Update FrameTree pipelines when appropriate

Instead of simply creating a new FrameTree when an iframe starts a
load, update the existing FrameTree's pipeline. This prevents the
FrameTree from accumulating many extra children.

4 files changed, 126 insertions, 95 deletions

diff --git a/components/compositing/constellation.rs b/components/compositing/constellation.rs
index 5bc565b7f9a..0024d516f43 100644
--- a/components/compositing/constellation.rs
+++ b/components/compositing/constellation.rs
@@ -98,7 +98,7 @@ struct FrameTree {
     /// The ID of this frame.
     pub id: FrameId,
     /// The pipeline for this frame.
-    pub pipeline: Rc<Pipeline>,
+    pub pipeline: RefCell<Rc<Pipeline>>,
     /// The parent frame's pipeline.
     pub parent: RefCell<Option<Rc<Pipeline>>>,
     /// A vector of child frames.
@@ -112,7 +112,7 @@ impl FrameTree {
            -> FrameTree {
         FrameTree {
             id: id,
-            pipeline: pipeline,
+            pipeline: RefCell::new(pipeline.clone()),
             parent: RefCell::new(parent_pipeline),
             children: RefCell::new(vec!()),
             has_compositor_layer: Cell::new(false),
@@ -165,7 +165,7 @@ pub struct FrameTreeDiff {
 impl FrameTree {
     fn to_sendable(&self) -> SendableFrameTree {
         SendableFrameTree {
-            pipeline: self.pipeline.to_sendable(),
+            pipeline: self.pipeline.borrow().to_sendable(),
             children: self.children
                           .borrow()
                           .iter()
@@ -178,18 +178,24 @@ impl FrameTree {
 trait FrameTreeTraversal {
     fn contains(&self, id: PipelineId) -> bool;
     fn find(&self, id: PipelineId) -> Option<Self>;
+    fn find_with_subpage_id(&self, id: Option<SubpageId>) -> Option<Rc<FrameTree>>;
     fn replace_child(&self, id: PipelineId, new_child: Self) -> ReplaceResult;
     fn iter(&self) -> FrameTreeIterator;
 }
 
 impl FrameTreeTraversal for Rc<FrameTree> {
     fn contains(&self, id: PipelineId) -> bool {
-        self.iter().any(|frame_tree| id == frame_tree.pipeline.id)
+        self.iter().any(|frame_tree| id == frame_tree.pipeline.borrow().id)
     }
 
     /// Returns the frame tree whose key is id
     fn find(&self, id: PipelineId) -> Option<Rc<FrameTree>> {
-        self.iter().find(|frame_tree| id == frame_tree.pipeline.id)
+        self.iter().find(|frame_tree| id == frame_tree.pipeline.borrow().id)
+    }
+
+    /// Returns the frame tree whose subpage is id
+    fn find_with_subpage_id(&self, id: Option<SubpageId>) -> Option<Rc<FrameTree>> {
+        self.iter().find(|frame_tree| id == frame_tree.pipeline.borrow().subpage_id)
     }
 
     /// Replaces a node of the frame tree in place. Returns the node that was removed or the
@@ -198,7 +204,7 @@ impl FrameTreeTraversal for Rc<FrameTree> {
         for frame_tree in self.iter() {
             let mut children = frame_tree.children.borrow_mut();
             let child = children.iter_mut()
-                .find(|child| child.frame_tree.pipeline.id == id);
+                .find(|child| child.frame_tree.pipeline.borrow().id == id);
             match child {
                 Some(child) => {
                     *new_child.parent.borrow_mut() = child.frame_tree.parent.borrow().clone();
@@ -293,7 +299,7 @@ impl NavigationContext {
     /// Loads a new set of page frames, returning all evicted frame trees
     fn load(&mut self, frame_tree: Rc<FrameTree>, compositor_proxy: &mut CompositorProxy)
             -> Vec<Rc<FrameTree>> {
-        debug!("navigating to {}", frame_tree.pipeline.id);
+        debug!("navigating to {}", frame_tree.pipeline.borrow().id);
         let evicted = replace(&mut self.next, vec!());
         match self.current.take() {
             Some(current) => self.previous.push(current),
@@ -334,7 +340,7 @@ impl NavigationContext {
         self.current = Some(new_frame.clone());
         compositor_proxy.send(CompositorMsg::ChangePageLoadData(
             new_frame.id,
-            new_frame.pipeline.load_data.clone()));
+            new_frame.pipeline.borrow().load_data.clone()));
     }
 }
 
@@ -463,11 +469,12 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                 debug!("constellation got frame rect message");
                 self.handle_frame_rect_msg(pipeline_id, subpage_id, Rect::from_untyped(&rect));
             }
-            ConstellationMsg::ScriptLoadedURLInIFrame(url, source_pipeline_id, subpage_id, sandbox) => {
+            ConstellationMsg::ScriptLoadedURLInIFrame(url, source_pipeline_id, new_subpage_id, old_subpage_id, sandbox) => {
                 debug!("constellation got iframe URL load message");
                 self.handle_script_loaded_url_in_iframe_msg(url,
                                                             source_pipeline_id,
-                                                            subpage_id,
+                                                            new_subpage_id,
+                                                            old_subpage_id,
                                                             sandbox);
             }
             ConstellationMsg::SetCursor(cursor) => self.handle_set_cursor_msg(cursor),
@@ -550,12 +557,12 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
 
         loop {
             let idx = self.pending_frames.iter().position(|pending| {
-                pending.after.pipeline.id == pipeline_id
+                pending.after.pipeline.borrow().id == pipeline_id
             });
             match idx {
                 Some(idx) => {
                     debug!("removing pending frame change for failed pipeline");
-                    self.pending_frames[idx].after.pipeline.force_exit();
+                    self.pending_frames[idx].after.pipeline.borrow().force_exit();
                     self.compositor_proxy.send(CompositorMsg::PaintTaskExited(old_pipeline.id));
                     self.pending_frames.remove(idx);
                 },
@@ -607,7 +614,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
 
         // Returns true if a child frame tree's subpage id matches the given subpage id
         let subpage_eq = |child_frame_tree: & &mut ChildFrameTree| {
-            child_frame_tree.frame_tree.pipeline.
+            child_frame_tree.frame_tree.pipeline.borrow().
                 subpage_id.expect("Constellation:
                 child frame does not have a subpage id. This should not be possible.")
                 == subpage_id
@@ -668,7 +675,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                              device_pixel_ratio: ScaleFactor<ViewportPx,DevicePixel,f32>) {
             child_frame_tree.rect = Some(rect);
             // NOTE: work around borrowchk issues
-            let pipeline = &child_frame_tree.frame_tree.pipeline;
+            let pipeline = &*child_frame_tree.frame_tree.pipeline.borrow();
             if !already_sent.contains(&pipeline.id) {
                 if is_active {
                     let ScriptControlChan(ref script_chan) = pipeline.script_chan;
@@ -688,31 +695,60 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         }
     }
 
+    fn update_child_pipeline(frame_tree: Rc<FrameTree>,
+                             new_pipeline: Rc<Pipeline>,
+                             old_subpage_id: SubpageId) {
+        let existing_tree = match frame_tree.find_with_subpage_id(Some(old_subpage_id)) {
+            Some(existing_tree) => existing_tree.clone(),
+            None => panic!("Tried to update non-existing frame tree with pipeline={} subpage={}",
+                           new_pipeline.id,
+                           old_subpage_id),
+        };
+
+        *existing_tree.pipeline.borrow_mut() = new_pipeline;
+    }
+
+    fn create_or_update_child_pipeline(&mut self,
+                                       frame_tree: Rc<FrameTree>,
+                                       new_pipeline: Rc<Pipeline>,
+                                       new_rect: Option<TypedRect<PagePx, f32>>,
+                                       old_subpage_id: Option<SubpageId>) {
+        match old_subpage_id {
+            Some(old_subpage_id) =>
+                Constellation::update_child_pipeline(frame_tree.clone(), new_pipeline, old_subpage_id),
+            None => {
+                let new_frame_tree =
+                     Rc::new(FrameTree::new(self.get_next_frame_id(), new_pipeline,
+                                            Some(frame_tree.pipeline.borrow().clone())));
+                frame_tree.children.borrow_mut().push(ChildFrameTree::new(new_frame_tree,
+                                                                          new_rect));
+            }
+        }
+    }
 
     // The script task associated with pipeline_id has loaded a URL in an iframe via script. This
     // will result in a new pipeline being spawned and a frame tree being added to
-    // source_pipeline_id's frame tree's children. This message is never the result of a page
-    // navigation.
+    // containing_page_pipeline_id's frame tree's children. This message is never the result of a
+    // page navigation.
     fn handle_script_loaded_url_in_iframe_msg(&mut self,
                                               url: Url,
-                                              source_pipeline_id: PipelineId,
-                                              subpage_id: SubpageId,
+                                              containing_page_pipeline_id: PipelineId,
+                                              new_subpage_id: SubpageId,
+                                              old_subpage_id: Option<SubpageId>,
                                               sandbox: IFrameSandboxState) {
         // Start by finding the frame trees matching the pipeline id,
         // and add the new pipeline to their sub frames.
-        let frame_trees = self.find_all(source_pipeline_id);
+        let frame_trees = self.find_all(containing_page_pipeline_id);
         if frame_trees.is_empty() {
             panic!("Constellation: source pipeline id of ScriptLoadedURLInIFrame is not in
                     navigation context, nor is it in a pending frame. This should be
                     impossible.");
         }
 
-        let next_pipeline_id = self.get_next_pipeline_id();
-
         // Compare the pipeline's url to the new url. If the origin is the same,
         // then reuse the script task in creating the new pipeline
-        let source_pipeline = self.pipelines.get(&source_pipeline_id).expect("Constellation:
-            source Id of ScriptLoadedURLInIFrame does have an associated pipeline in
+        let source_pipeline = self.pipelines.get(&containing_page_pipeline_id).expect("Constellation:
+            source Id of ScriptLoadedURLInIFrameMsg does have an associated pipeline in
             constellation. This should be impossible.").clone();
 
         let source_url = source_pipeline.load_data.url.clone();
@@ -730,21 +766,20 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
             None
         };
 
+        let new_frame_pipeline_id = self.get_next_pipeline_id();
         let pipeline = self.new_pipeline(
-            next_pipeline_id,
-            Some(subpage_id),
+            new_frame_pipeline_id,
+            Some(new_subpage_id),
             script_pipeline,
             LoadData::new(url)
         );
 
-        let rect = self.pending_sizes.remove(&(source_pipeline_id, subpage_id));
+        let rect = self.pending_sizes.remove(&(containing_page_pipeline_id, new_subpage_id));
         for frame_tree in frame_trees.iter() {
-            let next_frame_id = self.get_next_frame_id();
-            frame_tree.children.borrow_mut().push(ChildFrameTree::new(
-                Rc::new(FrameTree::new(next_frame_id,
-                                       pipeline.clone(),
-                                       Some(source_pipeline.clone()))),
-                rect));
+            self.create_or_update_child_pipeline(frame_tree.clone(),
+                                                 pipeline.clone(),
+                                                 rect,
+                                                 old_subpage_id);
         }
         self.pipelines.insert(pipeline.id, pipeline);
     }
@@ -778,7 +813,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         // changes would be overridden by changing the subframe associated with source_id.
 
         let parent = source_frame.parent.clone();
-        let subpage_id = source_frame.pipeline.subpage_id;
+        let subpage_id = source_frame.pipeline.borrow().subpage_id;
         let next_pipeline_id = self.get_next_pipeline_id();
         let next_frame_id = self.get_next_frame_id();
         let pipeline = self.new_pipeline(next_pipeline_id, subpage_id, None, load_data);
@@ -805,7 +840,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                 } else {
                     let old = self.current_frame().as_ref().unwrap();
                     for frame in old.iter() {
-                        frame.pipeline.revoke_paint_permission();
+                        frame.pipeline.borrow().revoke_paint_permission();
                     }
                 }
                 self.navigation_context.forward(&mut *self.compositor_proxy)
@@ -817,7 +852,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                 } else {
                     let old = self.current_frame().as_ref().unwrap();
                     for frame in old.iter() {
-                        frame.pipeline.revoke_paint_permission();
+                        frame.pipeline.borrow().revoke_paint_permission();
                     }
                 }
                 self.navigation_context.back(&mut *self.compositor_proxy)
@@ -825,7 +860,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         };
 
         for frame in destination_frame.iter() {
-            frame.pipeline.load();
+            frame.pipeline.borrow().load();
         }
         self.grant_paint_permission(destination_frame, NavigationType::Navigate);
 
@@ -838,9 +873,9 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
 
     fn handle_key_msg(&self, key: Key, state: KeyState, mods: KeyModifiers) {
         self.current_frame().as_ref().map(|frame| {
-            let ScriptControlChan(ref chan) = frame.pipeline.script_chan;
+            let ScriptControlChan(ref chan) = frame.pipeline.borrow().script_chan;
             chan.send(ConstellationControlMsg::SendEvent(
-                frame.pipeline.id, CompositorEvent::KeyEvent(key, state, mods)));
+                frame.pipeline.borrow().id, CompositorEvent::KeyEvent(key, state, mods)));
         });
     }
 
@@ -871,7 +906,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         // If it is not found, it simply means that this pipeline will not receive
         // permission to paint.
         let pending_index = self.pending_frames.iter().rposition(|frame_change| {
-            frame_change.after.pipeline.id == pipeline_id
+            frame_change.after.pipeline.borrow().id == pipeline_id
         });
         match pending_index {
             Some(pending_index) => {
@@ -897,7 +932,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                             frame not contained in the current frame. This is a bug");
 
                         for frame in to_revoke.iter() {
-                            frame.pipeline.revoke_paint_permission();
+                            frame.pipeline.borrow().revoke_paint_permission();
                         }
 
                         // If to_add is not the root frame, then replace revoked_frame with it.
@@ -907,8 +942,8 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                         {
                             if to_add.parent.borrow().is_some() {
                                 debug!("Constellation: replacing {} with {} in {}",
-                                       revoke_id, to_add.pipeline.id,
-                                       next_frame_tree.pipeline.id);
+                                       revoke_id, to_add.pipeline.borrow().id,
+                                       next_frame_tree.pipeline.borrow().id);
                                 flag = true;
                             }
                         }
@@ -921,7 +956,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                         // Add to_add to parent's children, if it is not the root
                         let parent = &to_add.parent;
                         for parent in parent.borrow().iter() {
-                            let subpage_id = to_add.pipeline.subpage_id
+                            let subpage_id = to_add.pipeline.borrow().subpage_id
                                 .expect("Constellation:
                                 Child frame's subpage id is None. This should be impossible.");
                             let rect = self.pending_sizes.remove(&(parent.id, subpage_id));
@@ -945,14 +980,14 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         let mut already_seen = HashSet::new();
         for frame_tree in self.current_frame().iter() {
             debug!("constellation sending resize message to active frame");
-            let pipeline = &frame_tree.pipeline;
+            let pipeline = &*frame_tree.pipeline.borrow();;
             let ScriptControlChan(ref chan) = pipeline.script_chan;
             let _ = chan.send_opt(ConstellationControlMsg::Resize(pipeline.id, new_size));
             already_seen.insert(pipeline.id);
         }
         for frame_tree in self.navigation_context.previous.iter()
             .chain(self.navigation_context.next.iter()) {
-            let pipeline = &frame_tree.pipeline;
+            let pipeline = &*frame_tree.pipeline.borrow();
             if !already_seen.contains(&pipeline.id) {
                 debug!("constellation sending resize message to inactive frame");
                 let ScriptControlChan(ref chan) = pipeline.script_chan;
@@ -967,9 +1002,10 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
             let frame_tree = &change.after;
             if frame_tree.parent.borrow().is_none() {
                 debug!("constellation sending resize message to pending outer frame ({})",
-                       frame_tree.pipeline.id);
-                let ScriptControlChan(ref chan) = frame_tree.pipeline.script_chan;
-                let _ = chan.send_opt(ConstellationControlMsg::Resize(frame_tree.pipeline.id, new_size));
+                       frame_tree.pipeline.borrow().id);
+                let ScriptControlChan(ref chan) = frame_tree.pipeline.borrow().script_chan;
+                let _ = chan.send_opt(ConstellationControlMsg::Resize(
+                    frame_tree.pipeline.borrow().id, new_size));
             }
         }
 
@@ -981,15 +1017,15 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         // TODO(tkuehn): should only exit once per unique script task,
         // and then that script task will handle sub-exits
         for frame_tree in frame_tree.iter() {
-            frame_tree.pipeline.exit(PipelineExitType::PipelineOnly);
-            self.compositor_proxy.send(CompositorMsg::PaintTaskExited(frame_tree.pipeline.id));
-            self.pipelines.remove(&frame_tree.pipeline.id);
+            frame_tree.pipeline.borrow().exit(PipelineExitType::PipelineOnly);
+            self.compositor_proxy.send(CompositorMsg::PaintTaskExited(frame_tree.pipeline.borrow().id));
+            self.pipelines.remove(&frame_tree.pipeline.borrow().id);
         }
     }
 
     fn handle_evicted_frames(&mut self, evicted: Vec<Rc<FrameTree>>) {
         for frame_tree in evicted.into_iter() {
-            if !self.navigation_context.contains(frame_tree.pipeline.id) {
+            if !self.navigation_context.contains(frame_tree.pipeline.borrow().id) {
                 self.close_pipelines(frame_tree);
             } else {
                 let frames = frame_tree.children.borrow().iter()
@@ -1030,7 +1066,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                 let mut iter = frame_tree.iter();
                 for frame in iter {
                     frame.has_compositor_layer.set(true);
-                    frame.pipeline.grant_paint_permission();
+                    frame.pipeline.borrow().grant_paint_permission();
                 }
             }
             Err(()) => {} // message has been discarded, probably shutting down
@@ -1043,7 +1079,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                                             -> Option<(ChildFrameTree, Rc<FrameTree>)> {
         for child in frame_tree.children.borrow().iter() {
             let child_frame_tree = child.frame_tree.clone();
-            if child.frame_tree.pipeline.id == child_pipeline_id {
+            if child.frame_tree.pipeline.borrow().id == child_pipeline_id {
                 return Some((ChildFrameTree::new(child_frame_tree, child.rect),
                             frame_tree.clone()));
             }
@@ -1070,13 +1106,13 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         };
 
         if child.frame_tree.has_compositor_layer.get() {
-            child.frame_tree.pipeline.grant_paint_permission();
+            child.frame_tree.pipeline.borrow().grant_paint_permission();
             return;
         }
 
         let sendable_frame_tree_diff = FrameTreeDiff {
-            parent_pipeline: parent.pipeline.to_sendable(),
-            pipeline: child.frame_tree.pipeline.to_sendable(),
+            parent_pipeline: parent.pipeline.borrow().to_sendable(),
+            pipeline: child.frame_tree.pipeline.borrow().to_sendable(),
             rect: child.rect,
         };
 
@@ -1085,7 +1121,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         match port.recv_opt() {
             Ok(()) => {
                 child.frame_tree.has_compositor_layer.set(true);
-                child.frame_tree.pipeline.grant_paint_permission();
+                child.frame_tree.pipeline.borrow().grant_paint_permission();
             }
             Err(()) => {} // The message has been discarded, we are probably shutting down.
         }
diff --git a/components/layout/wrapper.rs b/components/layout/wrapper.rs
index e11587710fa..98567dbcecd 100644
--- a/components/layout/wrapper.rs
+++ b/components/layout/wrapper.rs
@@ -144,8 +144,8 @@ pub trait TLayoutNode {
                     Some(elem) => elem,
                     None => panic!("not an iframe element!")
                 };
-            let size = (*iframe_element.unsafe_get()).size().unwrap();
-            (*size.pipeline_id(), *size.subpage_id())
+            ((*iframe_element.unsafe_get()).containing_page_pipeline_id().unwrap(),
+             (*iframe_element.unsafe_get()).subpage_id().unwrap())
         }
     }
 
diff --git a/components/msg/constellation_msg.rs b/components/msg/constellation_msg.rs
index cbd6062f724..5cd2a0cce39 100644
--- a/components/msg/constellation_msg.rs
+++ b/components/msg/constellation_msg.rs
@@ -203,7 +203,7 @@ pub enum Msg {
     LoadComplete,
     FrameRect(PipelineId, SubpageId, Rect<f32>),
     LoadUrl(PipelineId, LoadData),
-    ScriptLoadedURLInIFrame(Url, PipelineId, SubpageId, IFrameSandboxState),
+    ScriptLoadedURLInIFrame(Url, PipelineId, SubpageId, Option<SubpageId>, IFrameSandboxState),
     Navigate(NavigationDirection),
     PainterReady(PipelineId),
     ResizedWindow(WindowSizeData),
diff --git a/components/script/dom/htmliframeelement.rs b/components/script/dom/htmliframeelement.rs
index 683c18c678a..66d0f824fa2 100644
--- a/components/script/dom/htmliframeelement.rs
+++ b/components/script/dom/htmliframeelement.rs
@@ -20,7 +20,7 @@ use dom::node::{Node, NodeHelpers, NodeTypeId, window_from_node};
 use dom::urlhelper::UrlHelper;
 use dom::virtualmethods::VirtualMethods;
 use dom::window::Window;
-use page::IterablePage;
+use page::{IterablePage, Page};
 
 use servo_msg::constellation_msg::{PipelineId, SubpageId, ConstellationChan};
 use servo_msg::constellation_msg::IFrameSandboxState::{IFrameSandboxed, IFrameUnsandboxed};
@@ -44,7 +44,8 @@ enum SandboxAllowance {
 #[dom_struct]
 pub struct HTMLIFrameElement {
     htmlelement: HTMLElement,
-    size: Cell<Option<IFrameSize>>,
+    subpage_id: Cell<Option<SubpageId>>,
+    containing_page_pipeline_id: Cell<Option<PipelineId>>,
     sandbox: Cell<Option<u8>>,
 }
 
@@ -54,31 +55,12 @@ impl HTMLIFrameElementDerived for EventTarget {
     }
 }
 
-#[jstraceable]
-#[privatize]
-#[deriving(Copy)]
-pub struct IFrameSize {
-    pipeline_id: PipelineId,
-    subpage_id: SubpageId,
-}
-
-impl IFrameSize {
-    #[inline]
-    pub fn pipeline_id<'a>(&'a self) -> &'a PipelineId {
-        &self.pipeline_id
-    }
-
-    #[inline]
-    pub fn subpage_id<'a>(&'a self) -> &'a SubpageId {
-        &self.subpage_id
-    }
-}
-
 pub trait HTMLIFrameElementHelpers {
     fn is_sandboxed(self) -> bool;
     fn get_url(self) -> Option<Url>;
     /// http://www.whatwg.org/html/#process-the-iframe-attributes
     fn process_the_iframe_attributes(self);
+    fn generate_new_subpage_id(self, page: &Page) -> (SubpageId, Option<SubpageId>);
 }
 
 impl<'a> HTMLIFrameElementHelpers for JSRef<'a, HTMLIFrameElement> {
@@ -100,6 +82,13 @@ impl<'a> HTMLIFrameElementHelpers for JSRef<'a, HTMLIFrameElement> {
         })
     }
 
+    fn generate_new_subpage_id(self, page: &Page) -> (SubpageId, Option<SubpageId>) {
+        let old_subpage_id = self.subpage_id.get();
+        let subpage_id = page.get_next_subpage_id();
+        self.subpage_id.set(Some(subpage_id));
+        (subpage_id, old_subpage_id)
+    }
+
     fn process_the_iframe_attributes(self) {
         let url = match self.get_url() {
             Some(url) => url.clone(),
@@ -112,19 +101,19 @@ impl<'a> HTMLIFrameElementHelpers for JSRef<'a, HTMLIFrameElement> {
             IFrameUnsandboxed
         };
 
-        // Subpage Id
         let window = window_from_node(self).root();
         let window = window.r();
         let page = window.page();
-        let subpage_id = page.get_next_subpage_id();
+        let (new_subpage_id, old_subpage_id) = self.generate_new_subpage_id(page);
 
-        self.size.set(Some(IFrameSize {
-            pipeline_id: page.id,
-            subpage_id: subpage_id,
-        }));
+        self.containing_page_pipeline_id.set(Some(page.id));
 
         let ConstellationChan(ref chan) = page.constellation_chan;
-        chan.send(ConstellationMsg::ScriptLoadedURLInIFrame(url, page.id, subpage_id, sandboxed));
+    chan.send(ConstellationMsg::ScriptLoadedURLInIFrame(url,
+                                                        page.id,
+                                                        new_subpage_id,
+                                                        old_subpage_id,
+                                                        sandboxed));
     }
 }
 
@@ -132,7 +121,8 @@ impl HTMLIFrameElement {
     fn new_inherited(localName: DOMString, prefix: Option<DOMString>, document: JSRef<Document>) -> HTMLIFrameElement {
         HTMLIFrameElement {
             htmlelement: HTMLElement::new_inherited(HTMLElementTypeId::HTMLIFrameElement, localName, prefix, document),
-            size: Cell::new(None),
+            subpage_id: Cell::new(None),
+            containing_page_pipeline_id: Cell::new(None),
             sandbox: Cell::new(None),
         }
     }
@@ -144,8 +134,13 @@ impl HTMLIFrameElement {
     }
 
     #[inline]
-    pub fn size(&self) -> Option<IFrameSize> {
-        self.size.get()
+    pub fn containing_page_pipeline_id(&self) -> Option<PipelineId> {
+        self.containing_page_pipeline_id.get()
+    }
+
+    #[inline]
+    pub fn subpage_id(&self) -> Option<SubpageId> {
+        self.subpage_id.get()
     }
 }
 
@@ -171,11 +166,11 @@ impl<'a> HTMLIFrameElementMethods for JSRef<'a, HTMLIFrameElement> {
     }
 
     fn GetContentWindow(self) -> Option<Temporary<Window>> {
-        self.size.get().and_then(|size| {
+        self.subpage_id.get().and_then(|subpage_id| {
             let window = window_from_node(self).root();
             let children = window.page().children.borrow();
             let child = children.iter().find(|child| {
-                child.subpage_id.unwrap() == size.subpage_id
+                child.subpage_id.unwrap() == subpage_id
             });
             child.and_then(|page| {
                 page.frame.borrow().as_ref().map(|frame| {