Added a TopLevelBrowsingContextId type.

15 files changed, 404 insertions, 231 deletions

diff --git a/components/constellation/browsingcontext.rs b/components/constellation/browsingcontext.rs
index 5f6f29aecf6..a75ed4900bd 100644
--- a/components/constellation/browsingcontext.rs
+++ b/components/constellation/browsingcontext.rs
@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use euclid::size::TypedSize2D;
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
+use msg::constellation_msg::{BrowsingContextId, TopLevelBrowsingContextId, PipelineId};
 use pipeline::Pipeline;
 use script_traits::LoadData;
 use std::collections::HashMap;
@@ -23,6 +23,9 @@ pub struct BrowsingContext {
     /// The browsing context id.
     pub id: BrowsingContextId,
 
+    /// The top-level browsing context ancestor
+    pub top_level_id: TopLevelBrowsingContextId,
+
     /// The size of the frame.
     pub size: Option<TypedSize2D<f32, CSSPixel>>,
 
@@ -45,9 +48,15 @@ pub struct BrowsingContext {
 impl BrowsingContext {
     /// Create a new browsing context.
     /// Note this just creates the browsing context, it doesn't add it to the constellation's set of browsing contexts.
-    pub fn new(id: BrowsingContextId, pipeline_id: PipelineId, load_data: LoadData) -> BrowsingContext {
+    pub fn new(id: BrowsingContextId,
+               top_level_id: TopLevelBrowsingContextId,
+               pipeline_id: PipelineId,
+               load_data: LoadData)
+               -> BrowsingContext
+    {
         BrowsingContext {
             id: id,
+            top_level_id: top_level_id,
             size: None,
             pipeline_id: pipeline_id,
             instant: Instant::now(),
@@ -118,6 +127,9 @@ pub struct SessionHistoryChange {
     /// The browsing context to change.
     pub browsing_context_id: BrowsingContextId,
 
+    /// The top-level browsing context ancestor.
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
+
     /// The pipeline for the document being loaded.
     pub new_pipeline_id: PipelineId,
 
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index c16ff8c2049..7567b3d12cd 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -87,7 +87,7 @@ use ipc_channel::router::ROUTER;
 use itertools::Itertools;
 use layout_traits::LayoutThreadFactory;
 use log::{Log, LogLevel, LogLevelFilter, LogMetadata, LogRecord};
-use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId};
+use msg::constellation_msg::{BrowsingContextId, TopLevelBrowsingContextId, FrameType, PipelineId};
 use msg::constellation_msg::{Key, KeyModifiers, KeyState};
 use msg::constellation_msg::{PipelineNamespace, PipelineNamespaceId, TraversalDirection};
 use net_traits::{self, IpcSend, ResourceThreads};
@@ -227,14 +227,14 @@ pub struct Constellation<Message, LTF, STF> {
     /// event loop for each registered domain name (aka eTLD+1) in
     /// each top-level browsing context. We store the event loops in a map
     /// indexed by top-level browsing context id
-    /// (as a `BrowsingContextId`) and registered
+    /// (as a `TopLevelBrowsingContextId`) and registered
     /// domain name (as a `Host`) to event loops. This double
     /// indirection ensures that separate tabs do not share event
     /// loops, even if the same domain is loaded in each.
     /// It is important that scripts with the same eTLD+1
     /// share an event loop, since they can use `document.domain`
     /// to become same-origin, at which point they can share DOM objects.
-    event_loops: HashMap<BrowsingContextId, HashMap<Host, Weak<EventLoop>>>,
+    event_loops: HashMap<TopLevelBrowsingContextId, HashMap<Host, Weak<EventLoop>>>,
 
     /// The set of all the pipelines in the browser.
     /// (See the `pipeline` module for more details.)
@@ -251,7 +251,7 @@ pub struct Constellation<Message, LTF, STF> {
     pending_changes: Vec<SessionHistoryChange>,
 
     /// The root browsing context.
-    root_browsing_context_id: BrowsingContextId,
+    root_browsing_context_id: TopLevelBrowsingContextId,
 
     /// The currently focused pipeline for key events.
     focus_pipeline_id: Option<PipelineId>,
@@ -395,7 +395,7 @@ impl Log for FromScriptLogger {
     fn log(&self, record: &LogRecord) {
         if let Some(entry) = log_entry(record) {
             debug!("Sending log entry {:?}.", entry);
-            let top_level_id = BrowsingContextId::installed();
+            let top_level_id = TopLevelBrowsingContextId::installed();
             let thread_name = thread::current().name().map(ToOwned::to_owned);
             let msg = FromScriptMsg::LogEntry(top_level_id, thread_name, entry);
             let chan = self.constellation_chan.lock().unwrap_or_else(|err| err.into_inner());
@@ -433,7 +433,7 @@ impl Log for FromCompositorLogger {
     fn log(&self, record: &LogRecord) {
         if let Some(entry) = log_entry(record) {
             debug!("Sending log entry {:?}.", entry);
-            let top_level_id = BrowsingContextId::installed();
+            let top_level_id = TopLevelBrowsingContextId::installed();
             let thread_name = thread::current().name().map(ToOwned::to_owned);
             let msg = FromCompositorMsg::LogEntry(top_level_id, thread_name, entry);
             let chan = self.constellation_chan.lock().unwrap_or_else(|err| err.into_inner());
@@ -524,7 +524,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 pending_changes: vec!(),
                 // We initialize the namespace at 1, since we reserved namespace 0 for the constellation
                 next_pipeline_namespace_id: PipelineNamespaceId(1),
-                root_browsing_context_id: BrowsingContextId::new(),
+                root_browsing_context_id: TopLevelBrowsingContextId::new(),
                 focus_pipeline_id: None,
                 time_profiler_chan: state.time_profiler_chan,
                 mem_profiler_chan: state.mem_profiler_chan,
@@ -580,6 +580,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     fn new_pipeline(&mut self,
                     pipeline_id: PipelineId,
                     browsing_context_id: BrowsingContextId,
+                    top_level_browsing_context_id: TopLevelBrowsingContextId,
                     parent_info: Option<(PipelineId, FrameType)>,
                     initial_window_size: Option<TypedSize2D<f32, CSSPixel>>,
                     // TODO: we have to provide ownership of the LoadData
@@ -591,15 +592,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                     is_private: bool) {
         if self.shutting_down { return; }
 
-        // TODO: can we get a case where the child pipeline is created
-        // before the parent is part of the frame tree?
-        let top_level_id = match parent_info {
-            Some((_, FrameType::MozBrowserIFrame)) => browsing_context_id,
-            Some((parent_id, _)) => self.get_top_level_browsing_context_for_pipeline(parent_id),
-            None => self.root_browsing_context_id,
-        };
-
-        debug!("Creating new pipeline {} in top-level browsing context {}.", pipeline_id, top_level_id);
+        debug!("Creating new pipeline {} in browsing context {}.", pipeline_id, browsing_context_id);
 
         let (event_loop, host) = match sandbox {
             IFrameSandboxState::IFrameSandboxed => (None, None),
@@ -610,7 +603,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                     match reg_host(&load_data.url) {
                         None => (None, None),
                         Some(host) => {
-                            let event_loop = self.event_loops.get(&top_level_id)
+                            let event_loop = self.event_loops.get(&top_level_browsing_context_id)
                                 .and_then(|map| map.get(&host))
                                 .and_then(|weak| weak.upgrade());
                             match event_loop {
@@ -646,14 +639,6 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             .map(|pipeline| pipeline.visible)
             .or(parent_visibility);
 
-        // TODO: think about the case where the child pipeline is created
-        // before the parent is part of the frame tree.
-        let top_level_browsing_context_id = match parent_info {
-            Some((_, FrameType::MozBrowserIFrame)) => browsing_context_id,
-            Some((parent_id, _)) => self.get_top_level_browsing_context_for_pipeline(parent_id),
-            None => self.root_browsing_context_id,
-        };
-
         let result = Pipeline::spawn::<Message, LTF, STF>(InitialPipelineState {
             id: pipeline_id,
             browsing_context_id: browsing_context_id,
@@ -697,11 +682,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         self.pipelines.insert(pipeline_id, pipeline);
     }
 
-    /// Get an iterator for browsing contexts. Specify self.root_browsing context_id to
-    /// iterate the entire tree, or a specific browsing context id to iterate only that sub-tree.
-    /// Iterates over the fully active browsing contexts in the tree.
-    fn fully_active_browsing_contexts_iter(&self, browsing_context_id: BrowsingContextId)
-                                           -> FullyActiveBrowsingContextsIterator
+    /// Get an iterator for the fully active browsing contexts in a subtree.
+    fn fully_active_descendant_browsing_contexts_iter(&self, browsing_context_id: BrowsingContextId)
+                                                      -> FullyActiveBrowsingContextsIterator
     {
         FullyActiveBrowsingContextsIterator {
             stack: vec!(browsing_context_id),
@@ -710,11 +693,16 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
     }
 
-    /// Get an iterator for browsing contexts. Specify self.root_browsing_context_id to
-    /// iterate the entire tree, or a specific browsing context id to iterate only that sub-tree.
-    /// Iterates over all browsing contexts in the tree.
-    fn all_browsing_contexts_iter(&self, browsing_context_id: BrowsingContextId)
-                                  -> AllBrowsingContextsIterator
+    /// Get an iterator for the fully active browsing contexts in a tree.
+    fn fully_active_browsing_contexts_iter(&self, top_level_browsing_context_id: TopLevelBrowsingContextId)
+                                           -> FullyActiveBrowsingContextsIterator
+    {
+        self.fully_active_descendant_browsing_contexts_iter(BrowsingContextId::from(top_level_browsing_context_id))
+    }
+
+    /// Get an iterator for the browsing contexts in a subtree.
+    fn all_descendant_browsing_contexts_iter(&self, browsing_context_id: BrowsingContextId)
+                                             -> AllBrowsingContextsIterator
     {
         AllBrowsingContextsIterator {
             stack: vec!(browsing_context_id),
@@ -723,28 +711,35 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
     }
 
+    /// Get an iterator for the browsing contexts in a tree.
+    fn all_browsing_contexts_iter(&self, top_level_browsing_context_id: TopLevelBrowsingContextId)
+                                  -> AllBrowsingContextsIterator
+    {
+        self.all_descendant_browsing_contexts_iter(BrowsingContextId::from(top_level_browsing_context_id))
+    }
+
     /// The joint session future is the merge of the session future of every
     /// browsing_context, sorted chronologically.
-    fn joint_session_future<'a>(&'a self, browsing_context_id: BrowsingContextId)
+    fn joint_session_future<'a>(&'a self, top_level_browsing_context_id: TopLevelBrowsingContextId)
                                 -> impl Iterator<Item = &'a SessionHistoryEntry> + 'a
     {
-        self.all_browsing_contexts_iter(browsing_context_id)
+        self.all_browsing_contexts_iter(top_level_browsing_context_id)
             .map(|browsing_context| browsing_context.next.iter().rev())
             .kmerge_by(|a, b| a.instant.cmp(&b.instant) == Ordering::Less)
     }
 
     /// Is the joint session future empty?
-    fn joint_session_future_is_empty(&self, browsing_context_id: BrowsingContextId) -> bool {
-        self.all_browsing_contexts_iter(browsing_context_id)
+    fn joint_session_future_is_empty(&self, top_level_browsing_context_id: TopLevelBrowsingContextId) -> bool {
+        self.all_browsing_contexts_iter(top_level_browsing_context_id)
             .all(|browsing_context| browsing_context.next.is_empty())
     }
 
     /// The joint session past is the merge of the session past of every
     /// browsing_context, sorted reverse chronologically.
-    fn joint_session_past<'a>(&'a self, browsing_context_id: BrowsingContextId)
+    fn joint_session_past<'a>(&'a self, top_level_browsing_context_id: TopLevelBrowsingContextId)
                               -> impl Iterator<Item = &'a SessionHistoryEntry> + 'a
     {
-        self.all_browsing_contexts_iter(browsing_context_id)
+        self.all_browsing_contexts_iter(top_level_browsing_context_id)
             .map(|browsing_context| browsing_context.prev.iter().rev()
                  .scan(browsing_context.instant, |prev_instant, entry| {
                      let instant = *prev_instant;
@@ -756,17 +751,18 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     /// Is the joint session past empty?
-    fn joint_session_past_is_empty(&self, browsing_context_id: BrowsingContextId) -> bool {
-        self.all_browsing_contexts_iter(browsing_context_id)
+    fn joint_session_past_is_empty(&self, top_level_browsing_context_id: TopLevelBrowsingContextId) -> bool {
+        self.all_browsing_contexts_iter(top_level_browsing_context_id)
             .all(|browsing_context| browsing_context.prev.is_empty())
     }
 
     /// Create a new browsing context and update the internal bookkeeping.
     fn new_browsing_context(&mut self,
                  browsing_context_id: BrowsingContextId,
+                 top_level_id: TopLevelBrowsingContextId,
                  pipeline_id: PipelineId,
                  load_data: LoadData) {
-        let browsing_context = BrowsingContext::new(browsing_context_id, pipeline_id, load_data);
+        let browsing_context = BrowsingContext::new(browsing_context_id, top_level_id, pipeline_id, load_data);
         self.browsing_contexts.insert(browsing_context_id, browsing_context);
 
         // If a child browsing_context, add it to the parent pipeline.
@@ -1187,7 +1183,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
         // TODO: exit before the root browsing context is initialized?
         debug!("Removing root browsing context.");
-        let root_browsing_context_id = self.root_browsing_context_id;
+        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
         self.close_browsing_context(root_browsing_context_id, ExitPipelineMode::Normal);
 
         // Close any pending changes and pipelines
@@ -1289,12 +1285,17 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     fn handle_send_error(&mut self, pipeline_id: PipelineId, err: IpcError) {
         // Treat send error the same as receiving a panic message
         debug!("Pipeline {:?} send error ({}).", pipeline_id, err);
-        let top_level_browsing_context_id = self.get_top_level_browsing_context_for_pipeline(pipeline_id);
+        let top_level_browsing_context_id = self.pipelines.get(&pipeline_id)
+            .map(|pipeline| pipeline.browsing_context_id)
+            .and_then(|browsing_context_id| self.browsing_contexts.get(&browsing_context_id))
+            .map(|browsing_context| browsing_context.top_level_id)
+            .unwrap_or(self.root_browsing_context_id);
         let reason = format!("Send failed ({})", err);
         self.handle_panic(top_level_browsing_context_id, reason, None);
     }
 
-    fn handle_panic(&mut self, top_level_browsing_context_id: BrowsingContextId,
+    fn handle_panic(&mut self,
+                    top_level_browsing_context_id: TopLevelBrowsingContextId,
                     reason: String,
                     backtrace: Option<String>)
     {
@@ -1307,11 +1308,13 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
         debug!("Panic handler for top-level browsing context {}: {}.", top_level_browsing_context_id, reason);
 
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+
         // Notify the browser chrome that the pipeline has failed
         self.trigger_mozbrowsererror(top_level_browsing_context_id, reason, backtrace);
 
         let (window_size, pipeline_id) = {
-            let browsing_context = self.browsing_contexts.get(&top_level_browsing_context_id);
+            let browsing_context = self.browsing_contexts.get(&browsing_context_id);
             let window_size = browsing_context.and_then(|browsing_context| browsing_context.size);
             let pipeline_id = browsing_context.map(|browsing_context| browsing_context.pipeline_id);
             (window_size, pipeline_id)
@@ -1324,7 +1327,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             (pipeline_url, parent_info)
         };
 
-        self.close_browsing_context_children(top_level_browsing_context_id,
+        self.close_browsing_context_children(browsing_context_id,
                                              DiscardBrowsingContext::No,
                                              ExitPipelineMode::Force);
 
@@ -1341,10 +1344,11 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         let new_pipeline_id = PipelineId::new();
         let load_data = LoadData::new(failure_url, None, None, None);
         let sandbox = IFrameSandboxState::IFrameSandboxed;
-        self.new_pipeline(new_pipeline_id, top_level_browsing_context_id, parent_info,
+        self.new_pipeline(new_pipeline_id, browsing_context_id, top_level_browsing_context_id, parent_info,
                           window_size, load_data.clone(), sandbox, false);
         self.pending_changes.push(SessionHistoryChange {
-            browsing_context_id: top_level_browsing_context_id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
+            browsing_context_id: browsing_context_id,
             new_pipeline_id: new_pipeline_id,
             load_data: load_data,
             replace_instant: None,
@@ -1352,7 +1356,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn handle_log_entry(&mut self,
-                        top_level_browsing_context_id: Option<BrowsingContextId>,
+                        top_level_browsing_context_id: Option<TopLevelBrowsingContextId>,
                         thread_name: Option<String>,
                         entry: LogEntry)
     {
@@ -1391,6 +1395,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         let load_data = LoadData::new(url.clone(), None, None, None);
         let sandbox = IFrameSandboxState::IFrameUnsandboxed;
         self.new_pipeline(root_pipeline_id,
+                          BrowsingContextId::from(root_browsing_context_id),
                           root_browsing_context_id,
                           None,
                           Some(window_size),
@@ -1399,7 +1404,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                           false);
         self.handle_load_start_msg(root_pipeline_id);
         self.pending_changes.push(SessionHistoryChange {
-            browsing_context_id: self.root_browsing_context_id,
+            top_level_browsing_context_id: root_browsing_context_id,
+            browsing_context_id: BrowsingContextId::from(root_browsing_context_id),
             new_pipeline_id: root_pipeline_id,
             load_data: load_data,
             replace_instant: None,
@@ -1407,14 +1413,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn handle_iframe_size_msg(&mut self,
-                             iframe_sizes: Vec<(BrowsingContextId, TypedSize2D<f32, CSSPixel>)>) {
+                              iframe_sizes: Vec<(BrowsingContextId, TypedSize2D<f32, CSSPixel>)>) {
         for (browsing_context_id, size) in iframe_sizes {
             let window_size = WindowSizeData {
                 initial_viewport: size,
                 device_pixel_ratio: self.window_size.device_pixel_ratio,
             };
 
-            self.resize_frame(window_size, WindowSizeType::Initial, browsing_context_id);
+            self.resize_browsing_context(window_size, WindowSizeType::Initial, browsing_context_id);
         }
     }
 
@@ -1482,6 +1488,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
         // Create the new pipeline, attached to the parent and push to pending changes
         self.pending_changes.push(SessionHistoryChange {
+            top_level_browsing_context_id: load_info.info.top_level_browsing_context_id,
             browsing_context_id: load_info.info.browsing_context_id,
             new_pipeline_id: load_info.info.new_pipeline_id,
             load_data: load_data.clone(),
@@ -1490,6 +1497,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
         self.new_pipeline(load_info.info.new_pipeline_id,
                           load_info.info.browsing_context_id,
+                          load_info.info.top_level_browsing_context_id,
                           Some((load_info.info.parent_pipeline_id, load_info.info.frame_type)),
                           window_size,
                           load_data,
@@ -1506,6 +1514,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             frame_type,
             replace,
             browsing_context_id,
+            top_level_browsing_context_id,
             is_private,
         } = load_info;
 
@@ -1543,6 +1552,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         self.pipelines.insert(new_pipeline_id, pipeline);
 
         self.pending_changes.push(SessionHistoryChange {
+            top_level_browsing_context_id: top_level_browsing_context_id,
             browsing_context_id: browsing_context_id,
             new_pipeline_id: new_pipeline_id,
             load_data: load_data,
@@ -1596,13 +1606,16 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             let title = String::from("Alert");
             let return_value = String::from("");
             let event = MozBrowserEvent::ShowModalPrompt(prompt_type, title, message, return_value);
-            let top_level_browsing_context_id = self.get_top_level_browsing_context_for_pipeline(pipeline_id);
+            let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+            let top_level_browsing_context_id = self.pipelines.get(&pipeline_id)
+                .and_then(|pipeline| self.browsing_contexts.get(&pipeline.browsing_context_id))
+                .map(|browsing_context| browsing_context.top_level_id);
 
-            match self.browsing_contexts.get(&self.root_browsing_context_id) {
+            match self.browsing_contexts.get(&root_browsing_context_id) {
                 None => warn!("Alert sent after root browsing context closure."),
                 Some(root_browsing_context) => match self.pipelines.get(&root_browsing_context.pipeline_id) {
                     None => warn!("Alert sent after root pipeline closure."),
-                    Some(pipeline) => pipeline.trigger_mozbrowser_event(Some(top_level_browsing_context_id), event),
+                    Some(pipeline) => pipeline.trigger_mozbrowser_event(top_level_browsing_context_id, event),
                 }
             }
         }
@@ -1665,7 +1678,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
                 // Make sure no pending page would be overridden.
                 for change in &self.pending_changes {
-                    if change.browsing_context_id == root_browsing_context_id {
+                    if change.browsing_context_id == BrowsingContextId::from(root_browsing_context_id) {
                         // id that sent load msg is being changed already; abort
                         return None;
                     }
@@ -1684,7 +1697,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 // changes would be overridden by changing the subframe associated with source_id.
 
                 // Create the new pipeline
-                let window_size = self.browsing_contexts.get(&root_browsing_context_id)
+                let window_size = self.browsing_contexts.get(&BrowsingContextId::from(root_browsing_context_id))
                     .and_then(|browsing_context| browsing_context.size);
                 let new_pipeline_id = PipelineId::new();
                 let sandbox = IFrameSandboxState::IFrameUnsandboxed;
@@ -1694,12 +1707,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                     None
                 };
                 self.pending_changes.push(SessionHistoryChange {
-                    browsing_context_id: root_browsing_context_id,
+                    top_level_browsing_context_id: root_browsing_context_id,
+                    browsing_context_id: BrowsingContextId::from(root_browsing_context_id),
                     new_pipeline_id: new_pipeline_id,
                     load_data: load_data.clone(),
                     replace_instant: replace_instant,
                 });
                 self.new_pipeline(new_pipeline_id,
+                                  BrowsingContextId::from(root_browsing_context_id),
                                   root_browsing_context_id,
                                   None,
                                   window_size,
@@ -1735,7 +1750,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                                    pipeline_id: Option<PipelineId>,
                                    direction: TraversalDirection) {
         let top_level_browsing_context_id = pipeline_id
-            .map(|pipeline_id| self.get_top_level_browsing_context_for_pipeline(pipeline_id))
+            .and_then(|pipeline_id| self.pipelines.get(&pipeline_id))
+            .and_then(|pipeline| self.browsing_contexts.get(&pipeline.browsing_context_id))
+            .map(|browsing_context| browsing_context.top_level_id)
             .unwrap_or(self.root_browsing_context_id);
 
         let mut size = 0;
@@ -1768,11 +1785,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn handle_joint_session_history_length(&self, pipeline_id: PipelineId, sender: IpcSender<u32>) {
-        let browsing_context_id = self.get_top_level_browsing_context_for_pipeline(pipeline_id);
+        let top_level_browsing_context_id = self.pipelines.get(&pipeline_id)
+            .and_then(|pipeline| self.browsing_contexts.get(&pipeline.browsing_context_id))
+            .map(|browsing_context| browsing_context.top_level_id)
+            .unwrap_or(self.root_browsing_context_id);
 
         // Initialize length at 1 to count for the current active entry
         let mut length = 1;
-        for browsing_context in self.all_browsing_contexts_iter(browsing_context_id) {
+        for browsing_context in self.all_browsing_contexts_iter(top_level_browsing_context_id) {
             length += browsing_context.next.len();
             length += browsing_context.prev.len();
         }
@@ -1783,7 +1803,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         // Send to the explicitly focused pipeline (if it exists), or the root
         // browsing context's current pipeline. If neither exist, fall back to sending to
         // the compositor below.
-        let root_pipeline_id = self.browsing_contexts.get(&self.root_browsing_context_id)
+        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+        let root_pipeline_id = self.browsing_contexts.get(&root_browsing_context_id)
             .map(|root_browsing_context| root_browsing_context.pipeline_id);
         let pipeline_id = self.focus_pipeline_id.or(root_pipeline_id);
 
@@ -1808,7 +1829,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
     fn handle_reload_msg(&mut self) {
         // Send Reload constellation msg to root script channel.
-        let root_pipeline_id = self.browsing_contexts.get(&self.root_browsing_context_id)
+        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+        let root_pipeline_id = self.browsing_contexts.get(&root_browsing_context_id)
             .map(|root_browsing_context| root_browsing_context.pipeline_id);
 
         if let Some(pipeline_id) = root_pipeline_id {
@@ -1862,9 +1884,11 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         // and pass the event to that script thread.
         // If the pipeline lookup fails, it is because we have torn down the pipeline,
         // so it is reasonable to silently ignore the event.
-        let browsing_context_id = self.pipelines.get(&pipeline_id).map(|pipeline| pipeline.browsing_context_id);
+        let top_level_browsing_context_id = self.pipelines.get(&pipeline_id)
+            .and_then(|pipeline| self.browsing_contexts.get(&pipeline.browsing_context_id))
+            .map(|browsing_context| browsing_context.top_level_id);
         match self.pipelines.get(&parent_pipeline_id) {
-            Some(pipeline) => pipeline.trigger_mozbrowser_event(browsing_context_id, event),
+            Some(pipeline) => pipeline.trigger_mozbrowser_event(top_level_browsing_context_id, event),
             None => warn!("Pipeline {:?} handling mozbrowser event after closure.", parent_pipeline_id),
         }
     }
@@ -1872,7 +1896,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     fn handle_get_pipeline(&mut self,
                            browsing_context_id: Option<BrowsingContextId>,
                            resp_chan: IpcSender<Option<PipelineId>>) {
-        let browsing_context_id = browsing_context_id.unwrap_or(self.root_browsing_context_id);
+        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+        let browsing_context_id = browsing_context_id.unwrap_or(root_browsing_context_id);
         let current_pipeline_id = self.browsing_contexts.get(&browsing_context_id)
             .map(|browsing_context| browsing_context.pipeline_id);
         let pipeline_id_loaded = self.pending_changes.iter().rev()
@@ -1924,7 +1949,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn handle_remove_iframe_msg(&mut self, browsing_context_id: BrowsingContextId) -> Vec<PipelineId> {
-        let result = self.all_browsing_contexts_iter(browsing_context_id)
+        let result = self.all_descendant_browsing_contexts_iter(browsing_context_id)
             .flat_map(|browsing_context| browsing_context.next.iter().chain(browsing_context.prev.iter())
                       .filter_map(|entry| entry.pipeline_id)
                       .chain(once(browsing_context.pipeline_id)))
@@ -1939,7 +1964,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             None => return warn!("No browsing context associated with pipeline {:?}", pipeline_id),
         };
 
-        let child_pipeline_ids: Vec<PipelineId> = self.all_browsing_contexts_iter(browsing_context_id)
+        let child_pipeline_ids: Vec<PipelineId> = self.all_descendant_browsing_contexts_iter(browsing_context_id)
             .flat_map(|browsing_context| browsing_context.prev.iter().chain(browsing_context.next.iter())
                       .filter_map(|entry| entry.pipeline_id)
                       .chain(once(browsing_context.pipeline_id)))
@@ -2042,7 +2067,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 }
             },
             WebDriverCommandMsg::TakeScreenshot(pipeline_id, reply) => {
-                let current_pipeline_id = self.browsing_contexts.get(&self.root_browsing_context_id)
+                let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+                let current_pipeline_id = self.browsing_contexts.get(&root_browsing_context_id)
                     .map(|root_browsing_context| root_browsing_context.pipeline_id);
                 if Some(pipeline_id) == current_pipeline_id {
                     self.compositor_proxy.send(ToCompositorMsg::CreatePng(reply));
@@ -2071,16 +2097,25 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 let sandbox = IFrameSandboxState::IFrameUnsandboxed;
                 let new_pipeline_id = PipelineId::new();
                 let load_data = entry.load_data;
-                let (parent_info, window_size, is_private) = match self.browsing_contexts.get(&browsing_context_id) {
+                let (top_level_id, parent_info, window_size, is_private) =
+                    match self.browsing_contexts.get(&browsing_context_id)
+                {
                     Some(browsing_context) => match self.pipelines.get(&browsing_context.pipeline_id) {
-                        Some(pipeline) => (pipeline.parent_info, browsing_context.size, pipeline.is_private),
-                        None => (None, browsing_context.size, false),
+                        Some(pipeline) => (browsing_context.top_level_id,
+                                           pipeline.parent_info,
+                                           browsing_context.size,
+                                           pipeline.is_private),
+                        None => (browsing_context.top_level_id,
+                                 None,
+                                 browsing_context.size,
+                                 false),
                     },
                     None => return warn!("no browsing context to traverse"),
                 };
-                self.new_pipeline(new_pipeline_id, browsing_context_id, parent_info,
+                self.new_pipeline(new_pipeline_id, browsing_context_id, top_level_id, parent_info,
                                   window_size, load_data.clone(), sandbox, is_private);
                 self.pending_changes.push(SessionHistoryChange {
+                    top_level_browsing_context_id: top_level_id,
                     browsing_context_id: browsing_context_id,
                     new_pipeline_id: new_pipeline_id,
                     load_data: load_data,
@@ -2095,9 +2130,12 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         // frame tree is modified below.
         let update_focus_pipeline = self.focused_pipeline_is_descendant_of(entry.browsing_context_id);
 
-        let (old_pipeline_id, replaced_pipeline_id) = match self.browsing_contexts.get_mut(&browsing_context_id) {
+        let (old_pipeline_id, replaced_pipeline_id, top_level_id) =
+            match self.browsing_contexts.get_mut(&browsing_context_id)
+        {
             Some(browsing_context) => {
                 let old_pipeline_id = browsing_context.pipeline_id;
+                let top_level_id = browsing_context.top_level_id;
                 let mut curr_entry = browsing_context.current();
 
                 if entry.instant > browsing_context.instant {
@@ -2122,7 +2160,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
                 browsing_context.update_current(pipeline_id, entry);
 
-                (old_pipeline_id, replaced_pipeline_id)
+                (old_pipeline_id, replaced_pipeline_id, top_level_id)
             },
             None => return warn!("no browsing context to traverse"),
         };
@@ -2147,7 +2185,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         // Deactivate the old pipeline, and activate the new one.
         self.update_activity(old_pipeline_id);
         self.update_activity(pipeline_id);
-        self.notify_history_changed(pipeline_id);
+        self.notify_history_changed(top_level_id);
 
         // Set paint permissions correctly for the compositor layers.
         self.send_frame_tree();
@@ -2171,14 +2209,12 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
     }
 
-    fn notify_history_changed(&self, pipeline_id: PipelineId) {
+    fn notify_history_changed(&self, top_level_browsing_context_id: TopLevelBrowsingContextId) {
         // Send a flat projection of the history.
         // The final vector is a concatenation of the LoadData of the past entries,
         // the current entry and the future entries.
         // LoadData of inner frames are ignored and replaced with the LoadData of the parent.
 
-        let top_level_browsing_context_id = self.get_top_level_browsing_context_for_pipeline(pipeline_id);
-
         // Ignore LoadData of non-top-level browsing contexts.
         let keep_load_data_if_top_browsing_context = |entry: &SessionHistoryEntry| {
             match entry.pipeline_id {
@@ -2206,7 +2242,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             Some(load_data)
         };
 
-        let current_load_data = match self.browsing_contexts.get(&top_level_browsing_context_id) {
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+        let current_load_data = match self.browsing_contexts.get(&browsing_context_id) {
             Some(browsing_context) => browsing_context.load_data.clone(),
             None => return warn!("notify_history_changed error after top-level browsing context closed."),
         };
@@ -2229,27 +2266,6 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         self.compositor_proxy.send(ToCompositorMsg::HistoryChanged(entries, current_index));
     }
 
-    fn get_top_level_browsing_context_for_pipeline(&self, mut pipeline_id: PipelineId) -> BrowsingContextId {
-        if PREFS.is_mozbrowser_enabled() {
-            loop {
-                match self.pipelines.get(&pipeline_id) {
-                    Some(pipeline) => match pipeline.parent_info {
-                        Some((_, FrameType::MozBrowserIFrame)) => return pipeline.browsing_context_id,
-                        Some((parent_id, _)) => pipeline_id = parent_id,
-                        None => return self.root_browsing_context_id,
-                    },
-                    None => {
-                        warn!("Finding top-level ancestor for pipeline {} after closure.", pipeline_id);
-                        return self.root_browsing_context_id;
-                    },
-                }
-            }
-        } else {
-            // If mozbrowser is not enabled, the root browsing context is the only top-level browsing context
-            self.root_browsing_context_id
-        }
-    }
-
     fn load_url_for_webdriver(&mut self,
                               pipeline_id: PipelineId,
                               load_data: LoadData,
@@ -2301,10 +2317,11 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
         if new_context {
             self.new_browsing_context(change.browsing_context_id,
+                                      change.top_level_browsing_context_id,
                                       change.new_pipeline_id,
                                       change.load_data);
             self.update_activity(change.new_pipeline_id);
-            self.notify_history_changed(change.new_pipeline_id);
+            self.notify_history_changed(change.top_level_browsing_context_id);
         };
 
         if let Some(old_pipeline_id) = navigated {
@@ -2312,9 +2329,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             self.update_activity(old_pipeline_id);
             self.update_activity(change.new_pipeline_id);
             // Clear the joint session future
-            let top_level_id = self.get_top_level_browsing_context_for_pipeline(change.new_pipeline_id);
-            self.clear_joint_session_future(top_level_id);
-            self.notify_history_changed(change.new_pipeline_id);
+            self.clear_joint_session_future(change.top_level_browsing_context_id);
+            self.notify_history_changed(change.top_level_browsing_context_id);
         }
 
         if location_changed {
@@ -2356,8 +2372,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     fn handle_window_size_msg(&mut self, new_size: WindowSizeData, size_type: WindowSizeType) {
         debug!("handle_window_size_msg: {:?}", new_size.initial_viewport.to_untyped());
 
-        let browsing_context_id = self.root_browsing_context_id;
-        self.resize_frame(new_size, size_type, browsing_context_id);
+        let browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+        self.resize_browsing_context(new_size, size_type, browsing_context_id);
 
         if let Some(resize_channel) = self.webdriver.resize_channel.take() {
             let _ = resize_channel.send(new_size);
@@ -2386,7 +2402,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         //
         // If there is no root browsing context yet, the initial page has
         // not loaded, so there is nothing to save yet.
-        if !self.browsing_contexts.contains_key(&self.root_browsing_context_id) {
+        if !self.browsing_contexts.contains_key(&BrowsingContextId::from(self.root_browsing_context_id)) {
             return ReadyToSave::NoRootBrowsingContext;
         }
 
@@ -2530,12 +2546,12 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         self.set_activity(pipeline_id, self.get_activity(pipeline_id));
     }
 
-    /// Handle updating the size of a frame. This notifies every pipeline in the frame of the new
-    /// size.
-    fn resize_frame(&mut self,
-                    new_size: WindowSizeData,
-                    size_type: WindowSizeType,
-                    browsing_context_id: BrowsingContextId)
+    /// Handle updating the size of a browsing context.
+    /// This notifies every pipeline in the context of the new size.
+    fn resize_browsing_context(&mut self,
+                               new_size: WindowSizeData,
+                               size_type: WindowSizeType,
+                               browsing_context_id: BrowsingContextId)
     {
         if let Some(browsing_context) = self.browsing_contexts.get_mut(&browsing_context_id) {
             browsing_context.size = Some(new_size.initial_viewport);
@@ -2582,8 +2598,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
     }
 
-    fn clear_joint_session_future(&mut self, browsing_context_id: BrowsingContextId) {
-        let browsing_context_ids: Vec<BrowsingContextId> = self.all_browsing_contexts_iter(browsing_context_id)
+    fn clear_joint_session_future(&mut self, top_level_browsing_context_id: TopLevelBrowsingContextId) {
+        let browsing_context_ids: Vec<BrowsingContextId> =
+            self.all_browsing_contexts_iter(top_level_browsing_context_id)
             .map(|browsing_context| browsing_context.id)
             .collect();
         for browsing_context_id in browsing_context_ids {
@@ -2602,23 +2619,26 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     // Close a browsing context (and all children)
     fn close_browsing_context(&mut self, browsing_context_id: BrowsingContextId, exit_mode: ExitPipelineMode) {
         debug!("Closing browsing context {}.", browsing_context_id);
-        let parent_info = self.browsing_contexts.get(&browsing_context_id)
-            .and_then(|browsing_context| self.pipelines.get(&browsing_context.pipeline_id))
-            .and_then(|pipeline| pipeline.parent_info);
 
         self.close_browsing_context_children(browsing_context_id, DiscardBrowsingContext::Yes, exit_mode);
 
-        self.event_loops.remove(&browsing_context_id);
-        if self.browsing_contexts.remove(&browsing_context_id).is_none() {
-            warn!("Closing browsing context {:?} twice.", browsing_context_id);
+        let browsing_context = match self.browsing_contexts.remove(&browsing_context_id) {
+            None => return warn!("Closing browsing context {:?} twice.", browsing_context_id),
+            Some(browsing_context) => browsing_context,
+        };
+
+        if BrowsingContextId::from(browsing_context.top_level_id) == browsing_context_id {
+            self.event_loops.remove(&browsing_context.top_level_id);
         }
 
+        let parent_info = self.pipelines.get(&browsing_context.pipeline_id)
+            .and_then(|pipeline| pipeline.parent_info);
+
         if let Some((parent_pipeline_id, _)) = parent_info {
-            let parent_pipeline = match self.pipelines.get_mut(&parent_pipeline_id) {
+            match self.pipelines.get_mut(&parent_pipeline_id) {
                 None => return warn!("Pipeline {:?} child closed after parent.", parent_pipeline_id),
-                Some(parent_pipeline) => parent_pipeline,
+                Some(parent_pipeline) => parent_pipeline.remove_child(browsing_context_id),
             };
-            parent_pipeline.remove_child(browsing_context_id);
         }
         debug!("Closed browsing context {:?}.", browsing_context_id);
     }
@@ -2749,8 +2769,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         // Note that this function can panic, due to ipc-channel creation failure.
         // avoiding this panic would require a mechanism for dealing
         // with low-resource scenarios.
-        debug!("Sending frame tree for browsing context {}.", self.root_browsing_context_id);
-        if let Some(frame_tree) = self.browsing_context_to_sendable(self.root_browsing_context_id) {
+        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+        debug!("Sending frame tree for browsing context {}.", root_browsing_context_id);
+        if let Some(frame_tree) = self.browsing_context_to_sendable(root_browsing_context_id) {
             let (chan, port) = ipc::channel().expect("Failed to create IPC channel!");
             self.compositor_proxy.send(ToCompositorMsg::SetFrameTree(frame_tree,
                                                                      chan));
@@ -2768,11 +2789,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             Some(pipeline) => if let Some((parent_id, FrameType::MozBrowserIFrame)) = pipeline.parent_info {
                 match self.pipelines.get(&parent_id) {
                     Some(parent) => {
-                        let can_go_forward = !self.joint_session_future_is_empty(pipeline.browsing_context_id);
-                        let can_go_back = !self.joint_session_past_is_empty(pipeline.browsing_context_id);
+                        let top_level_browsing_context_id = self.browsing_contexts.get(&pipeline.browsing_context_id)
+                            .map(|browsing_context| browsing_context.top_level_id)
+                            .unwrap_or(self.root_browsing_context_id);
+                        let can_go_forward = !self.joint_session_future_is_empty(top_level_browsing_context_id);
+                        let can_go_back = !self.joint_session_past_is_empty(top_level_browsing_context_id);
                         let url = pipeline.url.to_string();
                         let event = MozBrowserEvent::LocationChange(url, can_go_back, can_go_forward);
-                        parent.trigger_mozbrowser_event(Some(pipeline.browsing_context_id), event);
+                        parent.trigger_mozbrowser_event(Some(top_level_browsing_context_id), event);
                     },
                     None => warn!("triggered mozbrowser location change on closed parent {}", parent_id),
                 }
@@ -2784,7 +2808,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     // https://developer.mozilla.org/en-US/docs/Web/Events/mozbrowsererror
     // Note that this does not require the pipeline to be an immediate child of the root
     fn trigger_mozbrowsererror(&mut self,
-                               top_level_browsing_context_id: BrowsingContextId,
+                               top_level_browsing_context_id: TopLevelBrowsingContextId,
                                reason: String,
                                backtrace: Option<String>)
     {
@@ -2808,8 +2832,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
 
         let event = MozBrowserEvent::Error(MozBrowserErrorType::Fatal, reason, report);
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
 
-        match self.browsing_contexts.get(&top_level_browsing_context_id) {
+        match self.browsing_contexts.get(&browsing_context_id) {
             None => warn!("Mozbrowser error after top-level browsing context closed."),
             Some(browsing_context) => match self.pipelines.get(&browsing_context.pipeline_id) {
                 None => warn!("Mozbrowser error after top-level pipeline closed."),
@@ -2826,7 +2851,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
     fn focused_pipeline_is_descendant_of(&self, browsing_context_id: BrowsingContextId) -> bool {
         self.focus_pipeline_id.map_or(false, |pipeline_id| {
-            self.fully_active_browsing_contexts_iter(browsing_context_id)
+            self.fully_active_descendant_browsing_contexts_iter(browsing_context_id)
                 .any(|browsing_context| browsing_context.pipeline_id == pipeline_id)
         })
     }
diff --git a/components/constellation/pipeline.rs b/components/constellation/pipeline.rs
index 01c386779bb..85fdc54289b 100644
--- a/components/constellation/pipeline.rs
+++ b/components/constellation/pipeline.rs
@@ -15,7 +15,7 @@ use ipc_channel::Error;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use ipc_channel::router::ROUTER;
 use layout_traits::LayoutThreadFactory;
-use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId, PipelineNamespaceId};
+use msg::constellation_msg::{BrowsingContextId, TopLevelBrowsingContextId, FrameType, PipelineId, PipelineNamespaceId};
 use net::image_cache::ImageCacheImpl;
 use net_traits::{IpcSend, ResourceThreads};
 use net_traits::image_cache::ImageCache;
@@ -104,7 +104,7 @@ pub struct InitialPipelineState {
     pub browsing_context_id: BrowsingContextId,
 
     /// The ID of the top-level browsing context that contains this Pipeline.
-    pub top_level_browsing_context_id: BrowsingContextId,
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
 
     /// The ID of the parent pipeline and frame type, if any.
     /// If `None`, this is the root.
@@ -201,6 +201,7 @@ impl Pipeline {
                     parent_info: state.parent_info,
                     new_pipeline_id: state.id,
                     browsing_context_id: state.browsing_context_id,
+                    top_level_browsing_context_id: state.top_level_browsing_context_id,
                     load_data: state.load_data,
                     window_size: window_size,
                     pipeline_port: pipeline_port,
@@ -393,7 +394,7 @@ impl Pipeline {
     /// This will cause an event to be fired on an iframe in the document,
     /// or on the `Window` if no frame is given.
     pub fn trigger_mozbrowser_event(&self,
-                                     child_id: Option<BrowsingContextId>,
+                                     child_id: Option<TopLevelBrowsingContextId>,
                                      event: MozBrowserEvent) {
         assert!(PREFS.is_mozbrowser_enabled());
 
@@ -433,8 +434,8 @@ impl Pipeline {
 #[derive(Deserialize, Serialize)]
 pub struct UnprivilegedPipelineContent {
     id: PipelineId,
+    top_level_browsing_context_id: TopLevelBrowsingContextId,
     browsing_context_id: BrowsingContextId,
-    top_level_browsing_context_id: BrowsingContextId,
     parent_info: Option<(PipelineId, FrameType)>,
     constellation_chan: IpcSender<ScriptMsg>,
     layout_to_constellation_chan: IpcSender<LayoutMsg>,
@@ -491,7 +492,7 @@ impl UnprivilegedPipelineContent {
         }, self.load_data.clone());
 
         LTF::create(self.id,
-                    Some(self.top_level_browsing_context_id),
+                    self.top_level_browsing_context_id,
                     self.load_data.url,
                     self.parent_info.is_some(),
                     layout_pair,
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 9f5b60b0045..4a3ceaab8f3 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -74,7 +74,8 @@ use layout::webrender_helpers::WebRenderDisplayListConverter;
 use layout::wrapper::LayoutNodeLayoutData;
 use layout::wrapper::drop_style_and_layout_data;
 use layout_traits::LayoutThreadFactory;
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
+use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::TopLevelBrowsingContextId;
 use net_traits::image_cache::{ImageCache, UsePlaceholder};
 use parking_lot::RwLock;
 use profile_traits::mem::{self, Report, ReportKind, ReportsChan};
@@ -130,6 +131,9 @@ pub struct LayoutThread {
     /// The ID of the pipeline that we belong to.
     id: PipelineId,
 
+    /// The ID of the top-level browsing context that we belong to.
+    top_level_browsing_context_id: TopLevelBrowsingContextId,
+
     /// The URL of the pipeline that we belong to.
     url: ServoUrl,
 
@@ -244,7 +248,7 @@ impl LayoutThreadFactory for LayoutThread {
 
     /// Spawns a new layout thread.
     fn create(id: PipelineId,
-              top_level_browsing_context_id: Option<BrowsingContextId>,
+              top_level_browsing_context_id: TopLevelBrowsingContextId,
               url: ServoUrl,
               is_iframe: bool,
               chan: (Sender<Msg>, Receiver<Msg>),
@@ -261,13 +265,13 @@ impl LayoutThreadFactory for LayoutThread {
         thread::Builder::new().name(format!("LayoutThread {:?}", id)).spawn(move || {
             thread_state::initialize(thread_state::LAYOUT);
 
-            if let Some(top_level_browsing_context_id) = top_level_browsing_context_id {
-                BrowsingContextId::install(top_level_browsing_context_id);
-            }
+            // In order to get accurate crash reports, we install the top-level bc id.
+            TopLevelBrowsingContextId::install(top_level_browsing_context_id);
 
             { // Ensures layout thread is destroyed before we send shutdown message
                 let sender = chan.0;
                 let layout = LayoutThread::new(id,
+                                               top_level_browsing_context_id,
                                                url,
                                                is_iframe,
                                                chan.1,
@@ -417,6 +421,7 @@ fn add_font_face_rules(stylesheet: &Stylesheet,
 impl LayoutThread {
     /// Creates a new `LayoutThread` structure.
     fn new(id: PipelineId,
+           top_level_browsing_context_id: TopLevelBrowsingContextId,
            url: ServoUrl,
            is_iframe: bool,
            port: Receiver<Msg>,
@@ -465,6 +470,7 @@ impl LayoutThread {
 
         LayoutThread {
             id: id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
             url: url,
             is_iframe: is_iframe,
             port: port,
@@ -732,7 +738,7 @@ impl LayoutThread {
 
     fn create_layout_thread(&self, info: NewLayoutThreadInfo) {
         LayoutThread::create(info.id,
-                             BrowsingContextId::installed(),
+                             self.top_level_browsing_context_id,
                              info.url.clone(),
                              info.is_parent,
                              info.layout_pair,
diff --git a/components/layout_traits/lib.rs b/components/layout_traits/lib.rs
index 53aa2529cb0..61fa3ad2449 100644
--- a/components/layout_traits/lib.rs
+++ b/components/layout_traits/lib.rs
@@ -20,7 +20,8 @@ extern crate webrender_traits;
 
 use gfx::font_cache_thread::FontCacheThread;
 use ipc_channel::ipc::{IpcReceiver, IpcSender};
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
+use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::TopLevelBrowsingContextId;
 use net_traits::image_cache::ImageCache;
 use profile_traits::{mem, time};
 use script_traits::{ConstellationControlMsg, LayoutControlMsg};
@@ -34,7 +35,7 @@ use std::sync::mpsc::{Receiver, Sender};
 pub trait LayoutThreadFactory {
     type Message;
     fn create(id: PipelineId,
-              top_level_browsing_context_id: Option<BrowsingContextId>,
+              top_level_browsing_context_id: TopLevelBrowsingContextId,
               url: ServoUrl,
               is_iframe: bool,
               chan: (Sender<Self::Message>, Receiver<Self::Message>),
diff --git a/components/msg/constellation_msg.rs b/components/msg/constellation_msg.rs
index edec5fb5231..44f23bc94b3 100644
--- a/components/msg/constellation_msg.rs
+++ b/components/msg/constellation_msg.rs
@@ -258,8 +258,6 @@ impl fmt::Display for PipelineId {
     }
 }
 
-thread_local!(pub static TOP_LEVEL_BROWSING_CONTEXT_ID: Cell<Option<BrowsingContextId>> = Cell::new(None));
-
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Copy, Hash, Debug, Deserialize, Serialize, HeapSizeOf)]
 pub struct BrowsingContextIndex(pub u32);
 
@@ -278,22 +276,45 @@ impl BrowsingContextId {
             new_browsing_context_id
         })
     }
+}
+
+impl fmt::Display for BrowsingContextId {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        let PipelineNamespaceId(namespace_id) = self.namespace_id;
+        let BrowsingContextIndex(index) = self.index;
+        write!(fmt, "({},{})", namespace_id, index)
+    }
+}
+
+thread_local!(pub static TOP_LEVEL_BROWSING_CONTEXT_ID: Cell<Option<TopLevelBrowsingContextId>> = Cell::new(None));
+
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Copy, Hash, Debug, Deserialize, Serialize, HeapSizeOf)]
+pub struct TopLevelBrowsingContextId(BrowsingContextId);
+
+impl TopLevelBrowsingContextId {
+    pub fn new() -> TopLevelBrowsingContextId {
+        TopLevelBrowsingContextId(BrowsingContextId::new())
+    }
     /// Each script and layout thread should have the top-level browsing context id installed,
     /// since it is used by crash reporting.
-    pub fn install(id: BrowsingContextId) {
+    pub fn install(id: TopLevelBrowsingContextId) {
         TOP_LEVEL_BROWSING_CONTEXT_ID.with(|tls| tls.set(Some(id)))
     }
 
-    pub fn installed() -> Option<BrowsingContextId> {
+    pub fn installed() -> Option<TopLevelBrowsingContextId> {
         TOP_LEVEL_BROWSING_CONTEXT_ID.with(|tls| tls.get())
     }
 }
 
-impl fmt::Display for BrowsingContextId {
+impl fmt::Display for TopLevelBrowsingContextId {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        let PipelineNamespaceId(namespace_id) = self.namespace_id;
-        let BrowsingContextIndex(index) = self.index;
-        write!(fmt, "({},{})", namespace_id, index)
+        self.0.fmt(fmt)
+    }
+}
+
+impl From<TopLevelBrowsingContextId> for BrowsingContextId {
+    fn from(id: TopLevelBrowsingContextId) -> BrowsingContextId {
+        id.0
     }
 }
 
diff --git a/components/script/dom/bindings/trace.rs b/components/script/dom/bindings/trace.rs
index 3b96d954dcb..dca1b0e2ea5 100644
--- a/components/script/dom/bindings/trace.rs
+++ b/components/script/dom/bindings/trace.rs
@@ -58,7 +58,7 @@ use js::glue::{CallObjectTracer, CallValueTracer};
 use js::jsapi::{GCTraceKindToAscii, Heap, JSObject, JSTracer, TraceKind};
 use js::jsval::JSVal;
 use js::rust::Runtime;
-use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId};
+use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId, TopLevelBrowsingContextId};
 use net_traits::{Metadata, NetworkError, ReferrerPolicy, ResourceThreads};
 use net_traits::filemanager_thread::RelativePos;
 use net_traits::image::base::{Image, ImageMetadata};
@@ -336,7 +336,8 @@ unsafe_no_jsmanaged_fields!(TrustedPromise);
 unsafe_no_jsmanaged_fields!(PropertyDeclarationBlock);
 // These three are interdependent, if you plan to put jsmanaged data
 // in one of these make sure it is propagated properly to containing structs
-unsafe_no_jsmanaged_fields!(DocumentActivity, BrowsingContextId, FrameType, WindowSizeData, WindowSizeType, PipelineId);
+unsafe_no_jsmanaged_fields!(DocumentActivity, WindowSizeData, WindowSizeType);
+unsafe_no_jsmanaged_fields!(BrowsingContextId, FrameType, PipelineId, TopLevelBrowsingContextId);
 unsafe_no_jsmanaged_fields!(TimerEventId, TimerSource);
 unsafe_no_jsmanaged_fields!(TimelineMarkerType);
 unsafe_no_jsmanaged_fields!(WorkerId);
diff --git a/components/script/dom/dedicatedworkerglobalscope.rs b/components/script/dom/dedicatedworkerglobalscope.rs
index 6b73df12dfc..f4de73747c4 100644
--- a/components/script/dom/dedicatedworkerglobalscope.rs
+++ b/components/script/dom/dedicatedworkerglobalscope.rs
@@ -27,7 +27,7 @@ use js::jsapi::{HandleValue, JS_SetInterruptCallback};
 use js::jsapi::{JSAutoCompartment, JSContext};
 use js::jsval::UndefinedValue;
 use js::rust::Runtime;
-use msg::constellation_msg::BrowsingContextId;
+use msg::constellation_msg::TopLevelBrowsingContextId;
 use net_traits::{IpcSend, load_whole_resource};
 use net_traits::request::{CredentialsMode, Destination, RequestInit, Type as RequestType};
 use script_runtime::{CommonScriptMsg, ScriptChan, ScriptPort, StackRootTLS, get_reports, new_rt_and_cx};
@@ -159,13 +159,13 @@ impl DedicatedWorkerGlobalScope {
                             closing: Arc<AtomicBool>) {
         let serialized_worker_url = worker_url.to_string();
         let name = format!("WebWorker for {}", serialized_worker_url);
-        let top_level_browsing_context_id = BrowsingContextId::installed();
+        let top_level_browsing_context_id = TopLevelBrowsingContextId::installed();
 
         thread::Builder::new().name(name).spawn(move || {
             thread_state::initialize(thread_state::SCRIPT | thread_state::IN_WORKER);
 
             if let Some(top_level_browsing_context_id) = top_level_browsing_context_id {
-                BrowsingContextId::install(top_level_browsing_context_id);
+                TopLevelBrowsingContextId::install(top_level_browsing_context_id);
             }
 
             let roots = RootCollection::new();
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index 3bca1dcee9e..378dee975fa 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -100,7 +100,7 @@ use ipc_channel::ipc::{self, IpcSender};
 use js::jsapi::{JSContext, JSObject, JSRuntime};
 use js::jsapi::JS_GetRuntime;
 use msg::constellation_msg::{ALT, CONTROL, SHIFT, SUPER};
-use msg::constellation_msg::{BrowsingContextId, Key, KeyModifiers, KeyState};
+use msg::constellation_msg::{BrowsingContextId, Key, KeyModifiers, KeyState, TopLevelBrowsingContextId};
 use net_traits::{FetchResponseMsg, IpcSend, ReferrerPolicy};
 use net_traits::CookieSource::NonHTTP;
 use net_traits::CoreResourceMsg::{GetCookiesForUrl, SetCookiesForUrl};
@@ -1899,7 +1899,21 @@ impl Document {
     /// Find an iframe element in the document.
     pub fn find_iframe(&self, browsing_context_id: BrowsingContextId) -> Option<Root<HTMLIFrameElement>> {
         self.iter_iframes()
-            .find(|node| node.browsing_context_id() == browsing_context_id)
+            .find(|node| node.browsing_context_id() == Some(browsing_context_id))
+    }
+
+    /// Find a mozbrowser iframe element in the document.
+    pub fn find_mozbrowser_iframe(&self,
+                                  top_level_browsing_context_id: TopLevelBrowsingContextId)
+                                  -> Option<Root<HTMLIFrameElement>>
+    {
+        match self.find_iframe(BrowsingContextId::from(top_level_browsing_context_id)) {
+            None => None,
+            Some(iframe) => {
+                assert!(iframe.Mozbrowser());
+                Some(iframe)
+            },
+        }
     }
 
     pub fn get_dom_loading(&self) -> u64 {
diff --git a/components/script/dom/htmliframeelement.rs b/components/script/dom/htmliframeelement.rs
index 8334b4cb421..00755213c75 100644
--- a/components/script/dom/htmliframeelement.rs
+++ b/components/script/dom/htmliframeelement.rs
@@ -40,7 +40,7 @@ use html5ever::{LocalName, Prefix};
 use ipc_channel::ipc;
 use js::jsapi::{JSAutoCompartment, JSContext, MutableHandleValue};
 use js::jsval::{NullValue, UndefinedValue};
-use msg::constellation_msg::{FrameType, BrowsingContextId, PipelineId, TraversalDirection};
+use msg::constellation_msg::{FrameType, BrowsingContextId, PipelineId, TopLevelBrowsingContextId, TraversalDirection};
 use net_traits::response::HttpsState;
 use script_layout_interface::message::ReflowQueryType;
 use script_thread::{ScriptThread, Runnable};
@@ -84,7 +84,8 @@ enum ProcessingMode {
 #[dom_struct]
 pub struct HTMLIFrameElement {
     htmlelement: HTMLElement,
-    browsing_context_id: BrowsingContextId,
+    top_level_browsing_context_id: Cell<Option<TopLevelBrowsingContextId>>,
+    browsing_context_id: Cell<Option<BrowsingContextId>>,
     pipeline_id: Cell<Option<PipelineId>>,
     pending_pipeline_id: Cell<Option<PipelineId>>,
     sandbox: MutNullableJS<DOMTokenList>,
@@ -112,13 +113,6 @@ impl HTMLIFrameElement {
         }).unwrap_or_else(|| ServoUrl::parse("about:blank").unwrap())
     }
 
-    pub fn generate_new_pipeline_id(&self) -> (Option<PipelineId>, PipelineId) {
-        let old_pipeline_id = self.pipeline_id.get();
-        let new_pipeline_id = PipelineId::new();
-        debug!("Frame {} created pipeline {}.", self.browsing_context_id, new_pipeline_id);
-        (old_pipeline_id, new_pipeline_id)
-    }
-
     pub fn navigate_or_reload_child_browsing_context(&self,
                                                      load_data: Option<LoadData>,
                                                      nav_type: NavigationType,
@@ -129,6 +123,16 @@ impl HTMLIFrameElement {
             IFrameUnsandboxed
         };
 
+        let browsing_context_id = match self.browsing_context_id() {
+            None => return warn!("Navigating unattached iframe."),
+            Some(id) => id,
+        };
+
+        let top_level_browsing_context_id = match self.top_level_browsing_context_id() {
+            None => return warn!("Navigating unattached iframe."),
+            Some(id) => id,
+        };
+
         let document = document_from_node(self);
 
         let mut load_blocker = self.load_blocker.borrow_mut();
@@ -144,7 +148,8 @@ impl HTMLIFrameElement {
         }
 
         let window = window_from_node(self);
-        let (old_pipeline_id, new_pipeline_id) = self.generate_new_pipeline_id();
+        let old_pipeline_id = self.pipeline_id();
+        let new_pipeline_id = PipelineId::new();
         self.pending_pipeline_id.set(Some(new_pipeline_id));
         let private_iframe = self.privatebrowsing();
         let frame_type = if self.Mozbrowser() { FrameType::MozBrowserIFrame } else { FrameType::IFrame };
@@ -152,7 +157,8 @@ impl HTMLIFrameElement {
         let global_scope = window.upcast::<GlobalScope>();
         let load_info = IFrameLoadInfo {
             parent_pipeline_id: global_scope.pipeline_id(),
-            browsing_context_id: self.browsing_context_id,
+            browsing_context_id: browsing_context_id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
             new_pipeline_id: new_pipeline_id,
             is_private: private_iframe,
             frame_type: frame_type,
@@ -171,7 +177,8 @@ impl HTMLIFrameElement {
                 let new_layout_info = NewLayoutInfo {
                     parent_info: Some((global_scope.pipeline_id(), frame_type)),
                     new_pipeline_id: new_pipeline_id,
-                    browsing_context_id: self.browsing_context_id,
+                    browsing_context_id: browsing_context_id,
+                    top_level_browsing_context_id: top_level_browsing_context_id,
                     load_data: load_data.unwrap(),
                     pipeline_port: pipeline_receiver,
                     content_process_shutdown_chan: None,
@@ -246,11 +253,29 @@ impl HTMLIFrameElement {
         // Synchronously create a new context and navigate it to about:blank.
         let url = ServoUrl::parse("about:blank").unwrap();
         let document = document_from_node(self);
-        let pipeline_id = Some(window_from_node(self).upcast::<GlobalScope>().pipeline_id());
+        let window = window_from_node(self);
+        let pipeline_id = Some(window.upcast::<GlobalScope>().pipeline_id());
         let load_data = LoadData::new(url, pipeline_id, document.get_referrer_policy(), Some(document.url().clone()));
+        let (browsing_context_id, top_level_browsing_context_id) = if self.Mozbrowser() {
+            let top_level_browsing_context_id = TopLevelBrowsingContextId::new();
+            (BrowsingContextId::from(top_level_browsing_context_id), top_level_browsing_context_id)
+        } else {
+            (BrowsingContextId::new(), window.window_proxy().top_level_browsing_context_id())
+        };
+        self.pipeline_id.set(None);
+        self.pending_pipeline_id.set(None);
+        self.top_level_browsing_context_id.set(Some(top_level_browsing_context_id));
+        self.browsing_context_id.set(Some(browsing_context_id));
         self.navigate_or_reload_child_browsing_context(Some(load_data), NavigationType::InitialAboutBlank, false);
     }
 
+    fn destroy_nested_browsing_context(&self) {
+        self.pipeline_id.set(None);
+        self.pending_pipeline_id.set(None);
+        self.top_level_browsing_context_id.set(None);
+        self.browsing_context_id.set(None);
+    }
+
     pub fn update_pipeline_id(&self, new_pipeline_id: PipelineId, reason: UpdatePipelineIdReason) {
         if self.pending_pipeline_id.get() != Some(new_pipeline_id) && reason == UpdatePipelineIdReason::Navigation {
             return;
@@ -277,7 +302,8 @@ impl HTMLIFrameElement {
                      document: &Document) -> HTMLIFrameElement {
         HTMLIFrameElement {
             htmlelement: HTMLElement::new_inherited(local_name, prefix, document),
-            browsing_context_id: BrowsingContextId::new(),
+            browsing_context_id: Cell::new(None),
+            top_level_browsing_context_id: Cell::new(None),
             pipeline_id: Cell::new(None),
             pending_pipeline_id: Cell::new(None),
             sandbox: Default::default(),
@@ -302,8 +328,13 @@ impl HTMLIFrameElement {
     }
 
     #[inline]
-    pub fn browsing_context_id(&self) -> BrowsingContextId {
-        self.browsing_context_id
+    pub fn browsing_context_id(&self) -> Option<BrowsingContextId> {
+        self.browsing_context_id.get()
+    }
+
+    #[inline]
+    pub fn top_level_browsing_context_id(&self) -> Option<TopLevelBrowsingContextId> {
+        self.top_level_browsing_context_id.get()
     }
 
     pub fn change_visibility_status(&self, visibility: bool) {
@@ -364,7 +395,7 @@ impl HTMLIFrameElement {
 
 pub trait HTMLIFrameElementLayoutMethods {
     fn pipeline_id(&self) -> Option<PipelineId>;
-    fn browsing_context_id(&self) -> BrowsingContextId;
+    fn browsing_context_id(&self) -> Option<BrowsingContextId>;
     fn get_width(&self) -> LengthOrPercentageOrAuto;
     fn get_height(&self) -> LengthOrPercentageOrAuto;
 }
@@ -380,9 +411,9 @@ impl HTMLIFrameElementLayoutMethods for LayoutJS<HTMLIFrameElement> {
 
     #[inline]
     #[allow(unsafe_code)]
-    fn browsing_context_id(&self) -> BrowsingContextId {
+    fn browsing_context_id(&self) -> Option<BrowsingContextId> {
         unsafe {
-            (*self.unsafe_get()).browsing_context_id
+            (*self.unsafe_get()).browsing_context_id.get()
         }
     }
 
@@ -541,7 +572,8 @@ impl HTMLIFrameElementMethods for HTMLIFrameElement {
 
     // https://html.spec.whatwg.org/multipage/#dom-iframe-contentwindow
     fn GetContentWindow(&self) -> Option<Root<WindowProxy>> {
-        self.pipeline_id.get().and_then(|_| ScriptThread::find_window_proxy(self.browsing_context_id))
+        self.browsing_context_id.get()
+            .and_then(|browsing_context_id| ScriptThread::find_window_proxy(browsing_context_id))
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-iframe-contentdocument
@@ -711,7 +743,7 @@ impl VirtualMethods for HTMLIFrameElement {
                 // is in a document tree and has a browsing context, which is what causes
                 // the child browsing context to be created.
                 if self.upcast::<Node>().is_in_doc_with_browsing_context() {
-                    debug!("iframe {} src set while in browsing context.", self.browsing_context_id);
+                    debug!("iframe src set while in browsing context.");
                     self.process_the_iframe_attributes(ProcessingMode::NotFirstTime);
                 }
             },
@@ -740,7 +772,7 @@ impl VirtualMethods for HTMLIFrameElement {
         // to the newly-created browsing context, and then process the
         // iframe attributes for the "first time"."
         if self.upcast::<Node>().is_in_doc_with_browsing_context() {
-            debug!("iframe {} bound to browsing context.", self.browsing_context_id);
+            debug!("iframe bound to browsing context.");
             debug_assert!(tree_in_doc, "is_in_doc_with_bc, but not tree_in_doc");
             self.create_nested_browsing_context();
             self.process_the_iframe_attributes(ProcessingMode::FirstTime);
@@ -754,13 +786,18 @@ impl VirtualMethods for HTMLIFrameElement {
         LoadBlocker::terminate(&mut blocker);
 
         // https://html.spec.whatwg.org/multipage/#a-browsing-context-is-discarded
-        debug!("Unbinding frame {}.", self.browsing_context_id);
         let window = window_from_node(self);
         let (sender, receiver) = ipc::channel().unwrap();
 
         // Ask the constellation to remove the iframe, and tell us the
         // pipeline ids of the closed pipelines.
-        let msg = ConstellationMsg::RemoveIFrame(self.browsing_context_id, sender);
+        let browsing_context_id = match self.browsing_context_id() {
+            None => return warn!("Unbinding already unbound iframe."),
+            Some(id) => id,
+        };
+        debug!("Unbinding frame {}.", browsing_context_id);
+
+        let msg = ConstellationMsg::RemoveIFrame(browsing_context_id, sender);
         window.upcast::<GlobalScope>().constellation_chan().send(msg).unwrap();
         let exited_pipeline_ids = receiver.recv().unwrap();
 
@@ -769,9 +806,11 @@ impl VirtualMethods for HTMLIFrameElement {
         // when the `PipelineExit` message arrives.
         for exited_pipeline_id in exited_pipeline_ids {
             if let Some(exited_document) = ScriptThread::find_document(exited_pipeline_id) {
+                debug!("Discarding browsing context for pipeline {}", exited_pipeline_id);
                 exited_document.window().window_proxy().discard_browsing_context();
                 for exited_iframe in exited_document.iter_iframes() {
-                    exited_iframe.pipeline_id.set(None);
+                    debug!("Discarding nested browsing context");
+                    exited_iframe.destroy_nested_browsing_context();
                 }
             }
         }
@@ -781,8 +820,7 @@ impl VirtualMethods for HTMLIFrameElement {
         // the load doesn't think that it's a navigation, but instead
         // a new iframe. Without this, the constellation gets very
         // confused.
-        self.pipeline_id.set(None);
-        self.pending_pipeline_id.set(None);
+        self.destroy_nested_browsing_context();
     }
 }
 
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index ff3e5d64787..cc817587eda 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -1136,7 +1136,7 @@ impl LayoutNodeHelpers for LayoutJS<Node> {
     fn iframe_browsing_context_id(&self) -> BrowsingContextId {
         let iframe_element = self.downcast::<HTMLIFrameElement>()
             .expect("not an iframe element!");
-        iframe_element.browsing_context_id()
+        iframe_element.browsing_context_id().unwrap()
     }
 
     fn iframe_pipeline_id(&self) -> PipelineId {
diff --git a/components/script/dom/windowproxy.rs b/components/script/dom/windowproxy.rs
index e022e69a810..2578bb3b46c 100644
--- a/components/script/dom/windowproxy.rs
+++ b/components/script/dom/windowproxy.rs
@@ -30,6 +30,7 @@ use js::jsval::{UndefinedValue, PrivateValue};
 use js::rust::get_object_class;
 use msg::constellation_msg::BrowsingContextId;
 use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::TopLevelBrowsingContextId;
 use std::cell::Cell;
 use std::ptr;
 
@@ -50,6 +51,10 @@ pub struct WindowProxy {
     /// of the container.
     browsing_context_id: BrowsingContextId,
 
+    /// The frame id of the top-level ancestor browsing context.
+    /// In the case that this is a top-level window, this is our id.
+    top_level_browsing_context_id: TopLevelBrowsingContextId,
+
     /// The pipeline id of the currently active document.
     /// May be None, when the currently active document is in another script thread.
     /// We do not try to keep the pipeline id for documents in other threads,
@@ -69,6 +74,7 @@ pub struct WindowProxy {
 
 impl WindowProxy {
     pub fn new_inherited(browsing_context_id: BrowsingContextId,
+                         top_level_browsing_context_id: TopLevelBrowsingContextId,
                          currently_active: Option<PipelineId>,
                          frame_element: Option<&Element>,
                          parent: Option<&WindowProxy>)
@@ -77,6 +83,7 @@ impl WindowProxy {
         WindowProxy {
             reflector: Reflector::new(),
             browsing_context_id: browsing_context_id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
             currently_active: Cell::new(currently_active),
             discarded: Cell::new(false),
             frame_element: frame_element.map(JS::from_ref),
@@ -87,6 +94,7 @@ impl WindowProxy {
     #[allow(unsafe_code)]
     pub fn new(window: &Window,
                browsing_context_id: BrowsingContextId,
+               top_level_browsing_context_id: TopLevelBrowsingContextId,
                frame_element: Option<&Element>,
                parent: Option<&WindowProxy>)
                -> Root<WindowProxy>
@@ -107,7 +115,11 @@ impl WindowProxy {
 
             // Create a new browsing context.
             let current = Some(window.global().pipeline_id());
-            let mut window_proxy = box WindowProxy::new_inherited(browsing_context_id, current, frame_element, parent);
+            let mut window_proxy = box WindowProxy::new_inherited(browsing_context_id,
+                                                                  top_level_browsing_context_id,
+                                                                  current,
+                                                                  frame_element,
+                                                                  parent);
 
             // The window proxy owns the browsing context.
             // When we finalize the window proxy, it drops the browsing context it owns.
@@ -126,6 +138,7 @@ impl WindowProxy {
     #[allow(unsafe_code)]
     pub fn new_dissimilar_origin(global_to_clone_from: &GlobalScope,
                                  browsing_context_id: BrowsingContextId,
+                                 top_level_browsing_context_id: TopLevelBrowsingContextId,
                                  parent: Option<&WindowProxy>)
                                  -> Root<WindowProxy>
     {
@@ -136,7 +149,11 @@ impl WindowProxy {
             let cx = global_to_clone_from.get_cx();
 
             // Create a new browsing context.
-            let mut window_proxy = box WindowProxy::new_inherited(browsing_context_id, None, None, parent);
+            let mut window_proxy = box WindowProxy::new_inherited(browsing_context_id,
+                                                                  top_level_browsing_context_id,
+                                                                  None,
+                                                                  None,
+                                                                  parent);
 
             // Create a new dissimilar-origin window.
             let window = DissimilarOriginWindow::new(global_to_clone_from, &*window_proxy);
@@ -175,6 +192,10 @@ impl WindowProxy {
         self.browsing_context_id
     }
 
+    pub fn top_level_browsing_context_id(&self) -> TopLevelBrowsingContextId {
+        self.top_level_browsing_context_id
+    }
+
     pub fn frame_element(&self) -> Option<&Element> {
         self.frame_element.r()
     }
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index eeab44c4c71..d66baaeda81 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -73,7 +73,7 @@ use js::jsval::UndefinedValue;
 use js::rust::Runtime;
 use mem::heap_size_of_self_and_children;
 use microtask::{MicrotaskQueue, Microtask};
-use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId, PipelineNamespace};
+use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId, PipelineNamespace, TopLevelBrowsingContextId};
 use net_traits::{CoreResourceMsg, FetchMetadata, FetchResponseListener};
 use net_traits::{IpcSend, Metadata, ReferrerPolicy, ResourceThreads};
 use net_traits::image_cache::{ImageCache, PendingImageResponse};
@@ -143,8 +143,10 @@ pub unsafe fn trace_thread(tr: *mut JSTracer) {
 struct InProgressLoad {
     /// The pipeline which requested this load.
     pipeline_id: PipelineId,
-    /// The frame being loaded into.
+    /// The browsing context being loaded into.
     browsing_context_id: BrowsingContextId,
+    /// The top level ancestor browsing context.
+    top_level_browsing_context_id: TopLevelBrowsingContextId,
     /// The parent pipeline and frame type associated with this load, if any.
     parent_info: Option<(PipelineId, FrameType)>,
     /// The current window size associated with this pipeline.
@@ -165,6 +167,7 @@ impl InProgressLoad {
     /// Create a new InProgressLoad object.
     fn new(id: PipelineId,
            browsing_context_id: BrowsingContextId,
+           top_level_browsing_context_id: TopLevelBrowsingContextId,
            parent_info: Option<(PipelineId, FrameType)>,
            layout_chan: Sender<message::Msg>,
            window_size: Option<WindowSizeData>,
@@ -173,6 +176,7 @@ impl InProgressLoad {
         InProgressLoad {
             pipeline_id: id,
             browsing_context_id: browsing_context_id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
             parent_info: parent_info,
             layout_chan: layout_chan,
             window_size: window_size,
@@ -548,11 +552,12 @@ impl ScriptThreadFactory for ScriptThread {
         thread::Builder::new().name(format!("ScriptThread {:?}", state.id)).spawn(move || {
             thread_state::initialize(thread_state::SCRIPT);
             PipelineNamespace::install(state.pipeline_namespace_id);
-            BrowsingContextId::install(state.top_level_browsing_context_id);
+            TopLevelBrowsingContextId::install(state.top_level_browsing_context_id);
             let roots = RootCollection::new();
             let _stack_roots_tls = StackRootTLS::new(&roots);
             let id = state.id;
             let browsing_context_id = state.browsing_context_id;
+            let top_level_browsing_context_id = state.top_level_browsing_context_id;
             let parent_info = state.parent_info;
             let mem_profiler_chan = state.mem_profiler_chan.clone();
             let window_size = state.window_size;
@@ -567,7 +572,7 @@ impl ScriptThreadFactory for ScriptThread {
             let mut failsafe = ScriptMemoryFailsafe::new(&script_thread);
 
             let origin = MutableOrigin::new(load_data.url.origin());
-            let new_load = InProgressLoad::new(id, browsing_context_id, parent_info,
+            let new_load = InProgressLoad::new(id, browsing_context_id, top_level_browsing_context_id, parent_info,
                                                layout_chan, window_size, load_data.url.clone(), origin);
             script_thread.start_page_load(new_load, load_data);
 
@@ -1117,10 +1122,10 @@ impl ScriptThread {
             ConstellationControlMsg::PostMessage(pipeline_id, origin, data) =>
                 self.handle_post_message_msg(pipeline_id, origin, data),
             ConstellationControlMsg::MozBrowserEvent(parent_pipeline_id,
-                                                     browsing_context_id,
+                                                     top_level_browsing_context_id,
                                                      event) =>
                 self.handle_mozbrowser_event_msg(parent_pipeline_id,
-                                                 browsing_context_id,
+                                                 top_level_browsing_context_id,
                                                  event),
             ConstellationControlMsg::UpdatePipelineId(parent_pipeline_id,
                                                       browsing_context_id,
@@ -1344,6 +1349,7 @@ impl ScriptThread {
             parent_info,
             new_pipeline_id,
             browsing_context_id,
+            top_level_browsing_context_id,
             load_data,
             window_size,
             pipeline_port,
@@ -1379,9 +1385,14 @@ impl ScriptThread {
         };
 
         // Kick off the fetch for the new resource.
-        let new_load = InProgressLoad::new(new_pipeline_id, browsing_context_id, parent_info,
-                                           layout_chan, window_size,
-                                           load_data.url.clone(), origin);
+        let new_load = InProgressLoad::new(new_pipeline_id,
+                                           browsing_context_id,
+                                           top_level_browsing_context_id,
+                                           parent_info,
+                                           layout_chan,
+                                           window_size,
+                                           load_data.url.clone(),
+                                           origin);
         if load_data.url.as_str() == "about:blank" {
             self.start_page_load_about_blank(new_load);
         } else {
@@ -1495,17 +1506,18 @@ impl ScriptThread {
     /// https://developer.mozilla.org/en-US/docs/Web/Events/mozbrowserloadstart
     fn handle_mozbrowser_event_msg(&self,
                                    parent_pipeline_id: PipelineId,
-                                   browsing_context_id: Option<BrowsingContextId>,
+                                   top_level_browsing_context_id: Option<TopLevelBrowsingContextId>,
                                    event: MozBrowserEvent) {
         let doc = match { self.documents.borrow().find_document(parent_pipeline_id) } {
             None => return warn!("Mozbrowser event after pipeline {} closed.", parent_pipeline_id),
             Some(doc) => doc,
         };
 
-        match browsing_context_id {
+        match top_level_browsing_context_id {
             None => doc.window().dispatch_mozbrowser_event(event),
-            Some(browsing_context_id) => match doc.find_iframe(browsing_context_id) {
-                None => warn!("Mozbrowser event after iframe {}/{} closed.", parent_pipeline_id, browsing_context_id),
+            Some(top_level_browsing_context_id) => match doc.find_mozbrowser_iframe(top_level_browsing_context_id) {
+                None => warn!("Mozbrowser event after iframe {}/{} closed.",
+                              parent_pipeline_id, top_level_browsing_context_id),
                 Some(frame_element) => frame_element.dispatch_mozbrowser_event(event),
             },
         }
@@ -1779,9 +1791,10 @@ impl ScriptThread {
     // construct a new dissimilar-origin browsing context, add it
     // to the `window_proxies` map, and return it.
     fn remote_window_proxy(&self,
-                               global_to_clone: &GlobalScope,
-                               pipeline_id: PipelineId)
-                               -> Option<Root<WindowProxy>>
+                           global_to_clone: &GlobalScope,
+                           top_level_browsing_context_id: TopLevelBrowsingContextId,
+                           pipeline_id: PipelineId)
+                           -> Option<Root<WindowProxy>>
     {
         let browsing_context_id = match self.ask_constellation_for_browsing_context_id(pipeline_id) {
             Some(browsing_context_id) => browsing_context_id,
@@ -1791,10 +1804,15 @@ impl ScriptThread {
             return Some(Root::from_ref(window_proxy));
         }
         let parent = match self.ask_constellation_for_parent_info(pipeline_id) {
-            Some((parent_id, FrameType::IFrame)) => self.remote_window_proxy(global_to_clone, parent_id),
+            Some((parent_id, FrameType::IFrame)) => self.remote_window_proxy(global_to_clone,
+                                                                             top_level_browsing_context_id,
+                                                                             parent_id),
             _ => None,
         };
-        let window_proxy = WindowProxy::new_dissimilar_origin(global_to_clone, browsing_context_id, parent.r());
+        let window_proxy = WindowProxy::new_dissimilar_origin(global_to_clone,
+                                                              browsing_context_id,
+                                                              top_level_browsing_context_id,
+                                                              parent.r());
         self.window_proxies.borrow_mut().insert(browsing_context_id, JS::from_ref(&*window_proxy));
         Some(window_proxy)
     }
@@ -1806,10 +1824,11 @@ impl ScriptThread {
     // construct a new similar-origin browsing context, add it
     // to the `window_proxies` map, and return it.
     fn local_window_proxy(&self,
-                              window: &Window,
-                              browsing_context_id: BrowsingContextId,
-                              parent_info: Option<(PipelineId, FrameType)>)
-                              -> Root<WindowProxy>
+                          window: &Window,
+                          browsing_context_id: BrowsingContextId,
+                          top_level_browsing_context_id: TopLevelBrowsingContextId,
+                          parent_info: Option<(PipelineId, FrameType)>)
+                          -> Root<WindowProxy>
     {
         if let Some(window_proxy) = self.window_proxies.borrow().get(&browsing_context_id) {
             window_proxy.set_currently_active(&*window);
@@ -1821,10 +1840,16 @@ impl ScriptThread {
         };
         let parent = match (parent_info, iframe.as_ref()) {
             (_, Some(iframe)) => Some(window_from_node(&**iframe).window_proxy()),
-            (Some((parent_id, FrameType::IFrame)), _) => self.remote_window_proxy(window.upcast(), parent_id),
+            (Some((parent_id, FrameType::IFrame)), _) => self.remote_window_proxy(window.upcast(),
+                                                                                  top_level_browsing_context_id,
+                                                                                  parent_id),
             _ => None,
         };
-        let window_proxy = WindowProxy::new(&window, browsing_context_id, iframe.r().map(Castable::upcast), parent.r());
+        let window_proxy = WindowProxy::new(&window,
+                                            browsing_context_id,
+                                            top_level_browsing_context_id,
+                                            iframe.r().map(Castable::upcast),
+                                            parent.r());
         self.window_proxies.borrow_mut().insert(browsing_context_id, JS::from_ref(&*window_proxy));
         window_proxy
     }
@@ -1882,7 +1907,10 @@ impl ScriptThread {
                                  self.webvr_thread.clone());
 
         // Initialize the browsing context for the window.
-        let window_proxy = self.local_window_proxy(&window, incomplete.browsing_context_id, incomplete.parent_info);
+        let window_proxy = self.local_window_proxy(&window,
+                                                   incomplete.browsing_context_id,
+                                                   incomplete.top_level_browsing_context_id,
+                                                   incomplete.parent_info);
         window.init_window_proxy(&window_proxy);
 
         let last_modified = metadata.headers.as_ref().and_then(|headers| {
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 889a06e5cb3..98e66ab8044 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -53,7 +53,7 @@ use hyper::header::Headers;
 use hyper::method::Method;
 use ipc_channel::ipc::{IpcReceiver, IpcSender};
 use libc::c_void;
-use msg::constellation_msg::{BrowsingContextId, FrameType, Key, KeyModifiers, KeyState};
+use msg::constellation_msg::{BrowsingContextId, TopLevelBrowsingContextId, FrameType, Key, KeyModifiers, KeyState};
 use msg::constellation_msg::{PipelineId, PipelineNamespaceId, TraversalDirection};
 use net_traits::{ReferrerPolicy, ResourceThreads};
 use net_traits::image::base::Image;
@@ -181,6 +181,8 @@ pub struct NewLayoutInfo {
     pub new_pipeline_id: PipelineId,
     /// Id of the browsing context associated with this pipeline.
     pub browsing_context_id: BrowsingContextId,
+    /// Id of the top-level browsing context associated with this pipeline.
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
     /// Network request data which will be initiated by the script thread.
     pub load_data: LoadData,
     /// Information about the initial window size.
@@ -260,9 +262,9 @@ pub enum ConstellationControlMsg {
     Navigate(PipelineId, BrowsingContextId, LoadData, bool),
     /// Post a message to a given window.
     PostMessage(PipelineId, Option<ImmutableOrigin>, Vec<u8>),
-    /// Requests the script thread forward a mozbrowser event to an iframe it owns,
+    /// Requests the script thread forward a mozbrowser event to a mozbrowser iframe it owns,
     /// or to the window if no browsing context id is provided.
-    MozBrowserEvent(PipelineId, Option<BrowsingContextId>, MozBrowserEvent),
+    MozBrowserEvent(PipelineId, Option<TopLevelBrowsingContextId>, MozBrowserEvent),
     /// Updates the current pipeline ID of a given iframe.
     /// First PipelineId is for the parent, second is the new PipelineId for the frame.
     UpdatePipelineId(PipelineId, BrowsingContextId, PipelineId, UpdatePipelineIdReason),
@@ -488,10 +490,10 @@ pub struct InitialScriptState {
     /// The subpage ID of this pipeline to create in its pipeline parent.
     /// If `None`, this is the root.
     pub parent_info: Option<(PipelineId, FrameType)>,
-    /// The ID of the frame this script is part of.
+    /// The ID of the browsing context this script is part of.
     pub browsing_context_id: BrowsingContextId,
-    /// The ID of the top-level frame this script is part of.
-    pub top_level_browsing_context_id: BrowsingContextId,
+    /// The ID of the top-level browsing context this script is part of.
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
     /// A channel with which messages can be sent to us (the script thread).
     pub control_chan: IpcSender<ConstellationControlMsg>,
     /// A port on which messages sent by the constellation to script can be received.
@@ -548,8 +550,11 @@ pub enum IFrameSandboxState {
 pub struct IFrameLoadInfo {
     /// Pipeline ID of the parent of this iframe
     pub parent_pipeline_id: PipelineId,
-    /// The ID for this iframe.
+    /// The ID for this iframe's nested browsing context.
     pub browsing_context_id: BrowsingContextId,
+    /// The ID for the top-level ancestor browsing context of this iframe's nested browsing context.
+    /// Note: this is the same as the browsing_context_id for mozbrowser iframes.
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
     /// The new pipeline ID that the iframe has generated.
     pub new_pipeline_id: PipelineId,
     ///  Whether this iframe should be considered private
@@ -761,7 +766,7 @@ pub enum ConstellationMsg {
     /// Reload the current page.
     Reload,
     /// A log entry, with the top-level browsing context id and thread name
-    LogEntry(Option<BrowsingContextId>, Option<String>, LogEntry),
+    LogEntry(Option<TopLevelBrowsingContextId>, Option<String>, LogEntry),
     /// Set the WebVR thread channel.
     SetWebVRThread(IpcSender<WebVRMsg>),
     /// Dispatch WebVR events to the subscribed script threads.
diff --git a/components/script_traits/script_msg.rs b/components/script_traits/script_msg.rs
index d34f2feeb5f..1ebe6c17b5f 100644
--- a/components/script_traits/script_msg.rs
+++ b/components/script_traits/script_msg.rs
@@ -17,7 +17,7 @@ use devtools_traits::{ScriptToDevtoolsControlMsg, WorkerId};
 use euclid::point::Point2D;
 use euclid::size::{Size2D, TypedSize2D};
 use ipc_channel::ipc::IpcSender;
-use msg::constellation_msg::{BrowsingContextId, FrameType, PipelineId, TraversalDirection};
+use msg::constellation_msg::{BrowsingContextId, TopLevelBrowsingContextId, FrameType, PipelineId, TraversalDirection};
 use msg::constellation_msg::{Key, KeyModifiers, KeyState};
 use net_traits::CoreResourceMsg;
 use net_traits::storage_thread::StorageType;
@@ -148,7 +148,7 @@ pub enum ScriptMsg {
     /// Script has handled a touch event, and either prevented or allowed default actions.
     TouchEventProcessed(EventResult),
     /// A log entry, with the top-level browsing context id and thread name
-    LogEntry(Option<BrowsingContextId>, Option<String>, LogEntry),
+    LogEntry(Option<TopLevelBrowsingContextId>, Option<String>, LogEntry),
     /// Notifies the constellation that this pipeline has exited.
     PipelineExited(PipelineId),
     /// Send messages from postMessage calls from serviceworker