auto merge of #4314 : mrobinson/servo/iframes-2, r=jdm

iframes added or loaded via script are not reflected visibly in the content of a page. The next step in making this happen is to have compositor layers accurately reflect newly recreated or loaded iframes. This change allows iframes to appear, but there are still some further changes necessary to make the output correct and reliable.

7 files changed, 212 insertions, 128 deletions

diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index a2d86d9d5a5..df0d505ad4e 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -5,7 +5,7 @@
 use compositor_layer::{CompositorData, CompositorLayer, WantsScrollEventsFlag};
 use compositor_task::{CompositorEventListener, CompositorProxy, CompositorReceiver};
 use compositor_task::{CompositorTask, LayerProperties, Msg};
-use constellation::{FrameId, FrameTreeDiff, SendableFrameTree};
+use constellation::{FrameId, SendableFrameTree};
 use pipeline::CompositionPipeline;
 use scrolling::ScrollingTimerProxy;
 use windowing;
@@ -289,9 +289,15 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.send_viewport_rects_for_all_layers();
             }
 
-            (Msg::FrameTreeUpdate(frame_tree_diff, response_channel),
+            (Msg::ChangeLayerPipelineAndRemoveChildren(old_pipeline, new_pipeline, response_channel),
              ShutdownState::NotShuttingDown) => {
-                self.update_frame_tree(&frame_tree_diff);
+                self.handle_change_layer_pipeline_and_remove_children(old_pipeline, new_pipeline);
+                response_channel.send(());
+            }
+
+            (Msg::CreateRootLayerForPipeline(parent_pipeline, pipeline, rect, response_channel),
+             ShutdownState::NotShuttingDown) => {
+                self.handle_create_root_layer_for_pipeline(parent_pipeline, pipeline, rect);
                 response_channel.send(());
             }
 
@@ -534,8 +540,41 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         return root_layer;
     }
 
-    fn update_frame_tree(&mut self, frame_tree_diff: &FrameTreeDiff) {
-        let pipeline_id = frame_tree_diff.parent_pipeline.id;
+    fn handle_change_layer_pipeline_and_remove_children(&mut self,
+                                                        old_pipeline: CompositionPipeline,
+                                                        new_pipeline: CompositionPipeline) {
+        let root_layer = match self.find_pipeline_root_layer(old_pipeline.id) {
+            Some(root_layer) => root_layer,
+            None => {
+                debug!("Ignoring ChangeLayerPipelineAndRemoveChildren message for pipeline ({}) shutting down.",
+                       old_pipeline.id);
+                return;
+            }
+        };
+
+        root_layer.clear_all_tiles(self);
+        root_layer.children().clear();
+
+        let new_pipeline_id = new_pipeline.id;
+        self.get_or_create_pipeline_details(new_pipeline_id).pipeline = Some(new_pipeline);
+    }
+
+    fn handle_create_root_layer_for_pipeline(&mut self,
+                                             parent_pipeline: CompositionPipeline,
+                                             new_pipeline: CompositionPipeline,
+                                             frame_rect: Option<TypedRect<PagePx, f32>>) {
+        let root_layer = self.create_root_layer_for_pipeline_and_rect(&new_pipeline, frame_rect);
+        match frame_rect {
+            Some(ref frame_rect) => {
+                *root_layer.masks_to_bounds.borrow_mut() = true;
+
+                let frame_rect = frame_rect.to_untyped();
+                *root_layer.bounds.borrow_mut() = Rect::from_untyped(&frame_rect);
+            }
+            None => {}
+        }
+
+        let pipeline_id = parent_pipeline.id;
         let parent_layer = match self.find_pipeline_root_layer(pipeline_id) {
             Some(root_layer) => root_layer,
             None => {
@@ -544,9 +583,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 return;
             }
         };
-        parent_layer.add_child(
-            self.create_root_layer_for_pipeline_and_rect(&frame_tree_diff.pipeline,
-                                                         frame_tree_diff.rect));
+        parent_layer.add_child(root_layer);
     }
 
     fn find_pipeline_root_layer(&self, pipeline_id: PipelineId) -> Option<Rc<Layer<CompositorData>>> {
@@ -683,7 +720,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             Some(ref layer) => {
                 layer.bounds.borrow_mut().origin = Point2D::from_untyped(&new_origin)
             }
-            None => panic!("Compositor received SetLayerOrigin for nonexistent layer"),
+            None => panic!("Compositor received SetLayerOrigin for nonexistent layer: {}", pipeline_id),
         };
 
         self.send_buffer_requests_for_all_layers();
diff --git a/components/compositing/compositor_task.rs b/components/compositing/compositor_task.rs
index aa4424e2214..3ff651da281 100644
--- a/components/compositing/compositor_task.rs
+++ b/components/compositing/compositor_task.rs
@@ -5,7 +5,7 @@
 //! Communication with the compositor task.
 
 pub use windowing;
-pub use constellation::{FrameId, SendableFrameTree, FrameTreeDiff};
+pub use constellation::{FrameId, SendableFrameTree};
 
 use compositor;
 use headless;
@@ -13,15 +13,17 @@ use windowing::{WindowEvent, WindowMethods};
 
 use azure::azure_hl::{SourceSurfaceMethods, Color};
 use geom::point::Point2D;
-use geom::rect::Rect;
+use geom::rect::{Rect, TypedRect};
 use geom::size::Size2D;
 use layers::platform::surface::{NativeCompositingGraphicsContext, NativeGraphicsMetadata};
 use layers::layers::LayerBufferSet;
+use pipeline::CompositionPipeline;
 use servo_msg::compositor_msg::{Epoch, LayerId, LayerMetadata, ReadyState};
 use servo_msg::compositor_msg::{PaintListener, PaintState, ScriptListener, ScrollPolicy};
 use servo_msg::constellation_msg::{ConstellationChan, LoadData, PipelineId};
 use servo_msg::constellation_msg::{Key, KeyState, KeyModifiers};
 use servo_util::cursor::Cursor;
+use servo_util::geometry::PagePx;
 use servo_util::memory::MemoryProfilerChan;
 use servo_util::time::TimeProfilerChan;
 use std::comm::{channel, Sender, Receiver};
@@ -206,8 +208,10 @@ pub enum Msg {
     PaintMsgDiscarded,
     /// Replaces the current frame tree, typically called during main frame navigation.
     SetFrameTree(SendableFrameTree, Sender<()>, ConstellationChan),
-    /// Sends an updated version of the frame tree.
-    FrameTreeUpdate(FrameTreeDiff, Sender<()>),
+    /// Requests the compositor to create a root layer for a new frame.
+    CreateRootLayerForPipeline(CompositionPipeline, CompositionPipeline, Option<TypedRect<PagePx, f32>>, Sender<()>),
+    /// Requests the compositor to change a root layer's pipeline and remove all child layers.
+    ChangeLayerPipelineAndRemoveChildren(CompositionPipeline, CompositionPipeline, Sender<()>),
     /// The load of a page has completed.
     LoadComplete,
     /// Indicates that the scrolling timeout with the given starting timestamp has happened and a
@@ -237,8 +241,9 @@ impl Show for Msg {
             Msg::ChangePageTitle(..) => write!(f, "ChangePageTitle"),
             Msg::ChangePageLoadData(..) => write!(f, "ChangePageLoadData"),
             Msg::PaintMsgDiscarded(..) => write!(f, "PaintMsgDiscarded"),
-            Msg::FrameTreeUpdate(..) => write!(f, "FrameTreeUpdate"),
             Msg::SetFrameTree(..) => write!(f, "SetFrameTree"),
+            Msg::CreateRootLayerForPipeline(..) => write!(f, "CreateRootLayerForPipeline"),
+            Msg::ChangeLayerPipelineAndRemoveChildren(..) => write!(f, "ChangeLayerPipelineAndRemoveChildren"),
             Msg::LoadComplete => write!(f, "LoadComplete"),
             Msg::ScrollTimeout(..) => write!(f, "ScrollTimeout"),
             Msg::KeyEvent(..) => write!(f, "KeyEvent"),
diff --git a/components/compositing/constellation.rs b/components/compositing/constellation.rs
index 5bc565b7f9a..fca671550b1 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.
@@ -112,12 +112,16 @@ 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),
         }
     }
+
+    fn add_child(&self, new_child: ChildFrameTree) {
+        self.children.borrow_mut().push(new_child);
+    }
 }
 
 #[deriving(Clone)]
@@ -152,20 +156,10 @@ enum ReplaceResult {
     OriginalNode(Rc<FrameTree>),
 }
 
-/// A struct that triggers the addition of a new frame to a previously existing frame tree.
-pub struct FrameTreeDiff {
-    /// The parent pipeline of the new frame.
-    pub parent_pipeline: CompositionPipeline,
-    /// The pipeline of the new frame itself.
-    pub pipeline: CompositionPipeline,
-    /// The frame rect of the new frame used for positioning its compositor layer.
-    pub rect: Option<TypedRect<PagePx, f32>>,
-}
-
 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 +172,24 @@ 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 +198,7 @@ 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 +293,7 @@ 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 +334,7 @@ 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 +463,12 @@ 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 +551,12 @@ 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 +608,7 @@ 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
@@ -625,9 +626,10 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                     match children.iter_mut().find(|child| subpage_eq(child)) {
                         None => {}
                         Some(child) => {
+                            let has_compositor_layer = child.frame_tree.has_compositor_layer.get();
                             update_child_rect(child,
                                               rect,
-                                              true,
+                                              has_compositor_layer,
                                               &mut already_sent,
                                               &mut self.compositor_proxy,
                                               self.window_size.device_pixel_ratio)
@@ -668,7 +670,7 @@ 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 +690,75 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         }
     }
 
+    fn update_child_pipeline(&mut self,
+                             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),
+        };
+
+        let old_pipeline = existing_tree.pipeline.borrow().clone();
+        *existing_tree.pipeline.borrow_mut() = new_pipeline.clone();
+
+        // If we have not yet sent this frame to the compositor for layer creation, we don't
+        // need to inform the compositor of updates to the pipeline.
+        if !existing_tree.has_compositor_layer.get() {
+            return;
+        }
+
+        let (chan, port) = channel();
+        self.compositor_proxy.send(CompositorMsg::ChangeLayerPipelineAndRemoveChildren(
+            old_pipeline.to_sendable(),
+            new_pipeline.to_sendable(),
+            chan));
+        let _ = port.recv_opt();
+    }
+
+    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) =>
+                self.update_child_pipeline(frame_tree.clone(), new_pipeline, old_subpage_id),
+            None => {
+                let child_tree = Rc::new(
+                    FrameTree::new(self.get_next_frame_id(),
+                                   new_pipeline,
+                                   Some(frame_tree.pipeline.borrow().clone())));
+                frame_tree.add_child(ChildFrameTree::new(child_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 +776,20 @@ 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 +823,7 @@ 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 +850,7 @@ 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 +862,7 @@ 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 +870,7 @@ 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 +883,9 @@ 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 +916,7 @@ 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 +942,7 @@ 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 +952,8 @@ 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;
                             }
                         }
@@ -921,15 +966,14 @@ 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));
                             let parent = next_frame_tree.find(parent.id).expect(
                                 "Constellation: pending frame has a parent frame that is not
                                 active. This is a bug.");
-                            parent.children.borrow_mut().push(ChildFrameTree::new(to_add.clone(),
-                                                              rect));
+                            parent.add_child(ChildFrameTree::new(to_add.clone(), rect));
                         }
                     }
                 }
@@ -945,14 +989,14 @@ 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 +1011,10 @@ 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 +1026,15 @@ 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 +1075,7 @@ 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 +1088,7 @@ 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()));
             }
@@ -1070,22 +1115,20 @@ 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(),
-            rect: child.rect,
-        };
-
         let (chan, port) = channel();
-        self.compositor_proxy.send(CompositorMsg::FrameTreeUpdate(sendable_frame_tree_diff, chan));
+        self.compositor_proxy.send(CompositorMsg::CreateRootLayerForPipeline(
+            parent.pipeline.borrow().to_sendable(),
+            child.frame_tree.pipeline.borrow().to_sendable(),
+            child.rect,
+            chan));
         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/compositing/headless.rs b/components/compositing/headless.rs
index 8b58d727292..86188636260 100644
--- a/components/compositing/headless.rs
+++ b/components/compositing/headless.rs
@@ -90,7 +90,11 @@ impl CompositorEventListener for NullCompositor {
                 response_chan.send(());
             }
 
-            Msg::FrameTreeUpdate(_, response_channel) => {
+            Msg::ChangeLayerPipelineAndRemoveChildren(_, _, response_channel) => {
+                response_channel.send(());
+            }
+
+            Msg::CreateRootLayerForPipeline(_, _, _, response_channel) => {
                 response_channel.send(());
             }
 
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| {