Merge in servo/master

1 files changed, 410 insertions, 291 deletions

diff --git a/components/script/script_task.rs b/components/script/script_task.rs
index 51361bd1436..673dc3510ef 100644
--- a/components/script/script_task.rs
+++ b/components/script/script_task.rs
@@ -3,7 +3,19 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 //! The script task is the task that owns the DOM in memory, runs JavaScript, and spawns parsing
-//! and layout tasks.
+//! and layout tasks. It's in charge of processing events for all same-origin pages in a frame
+//! tree, and manages the entire lifetime of pages in the frame tree from initial request to
+//! teardown.
+//!
+//! Page loads follow a two-step process. When a request for a new page load is received, the
+//! network request is initiated and the relevant data pertaining to the new page is stashed.
+//! While the non-blocking request is ongoing, the script task is free to process further events,
+//! noting when they pertain to ongoing loads (such as resizes/viewport adjustments). When the
+//! initial response is received for an ongoing load, the second phase starts - the frame tree
+//! entry is created, along with the Window and Document objects, and the appropriate parser
+//! takes over the response body. Once parsing is complete, the document lifecycle for loading
+//! a page runs its course and the script task returns to processing events in the main event
+//! loop.
 
 #![allow(unsafe_blocks)]
 
@@ -23,7 +35,7 @@ use dom::element::{Element, AttributeHandlers};
 use dom::event::{Event, EventHelpers};
 use dom::uievent::UIEvent;
 use dom::eventtarget::EventTarget;
-use dom::node::{self, Node, NodeHelpers, NodeDamage};
+use dom::node::{self, Node, NodeHelpers, NodeDamage, window_from_node};
 use dom::window::{Window, WindowHelpers, ScriptHelpers};
 use dom::worker::{Worker, TrustedWorkerAddress};
 use parse::html::{HTMLInput, parse_html};
@@ -49,19 +61,20 @@ use msg::constellation_msg::{LoadData, PipelineId, SubpageId};
 use msg::constellation_msg::{Failure, Msg, WindowSizeData, PipelineExitType};
 use msg::constellation_msg::Msg as ConstellationMsg;
 use net::image_cache_task::ImageCacheTask;
-use net::resource_task::{ResourceTask, ControlMsg};
+use net::resource_task::{ResourceTask, ControlMsg, LoadResponse};
 use net::resource_task::LoadData as NetLoadData;
 use net::storage_task::StorageTask;
 use string_cache::Atom;
 use util::geometry::to_frac_px;
 use util::smallvec::SmallVec;
 use util::str::DOMString;
-use util::task::spawn_named_with_send_on_failure;
+use util::task::{spawn_named, spawn_named_with_send_on_failure};
 use util::task_state;
 
+use geom::Rect;
 use geom::point::Point2D;
 use hyper::header::{LastModified, Headers};
-use js::jsapi::{JS_SetWrapObjectCallbacks, JS_SetGCZeal, JS_DEFAULT_ZEAL_FREQ, JS_GC};
+use js::jsapi::{JS_SetWrapObjectCallbacks, JS_SetGCZeal, JS_DEFAULT_ZEAL_FREQ};
 use js::jsapi::{JSContext, JSRuntime, JSObject};
 use js::jsapi::{JS_SetGCParameter, JSGC_MAX_BYTES};
 use js::jsapi::{JS_SetGCCallback, JSGCStatus, JSGC_BEGIN, JSGC_END};
@@ -73,7 +86,6 @@ use libc;
 use std::any::Any;
 use std::borrow::ToOwned;
 use std::cell::Cell;
-use std::mem::replace;
 use std::num::ToPrimitive;
 use std::rc::Rc;
 use std::result::Result;
@@ -83,6 +95,43 @@ use time::Tm;
 
 thread_local!(pub static STACK_ROOTS: Cell<Option<RootCollectionPtr>> = Cell::new(None));
 
+/// A document load that is in the process of fetching the requested resource. Contains
+/// data that will need to be present when the document and frame tree entry are created,
+/// but is only easily available at initiation of the load and on a push basis (so some
+/// data will be updated according to future resize events, viewport changes, etc.)
+struct InProgressLoad {
+    /// The pipeline which requested this load.
+    pipeline_id: PipelineId,
+    /// The parent pipeline and child subpage associated with this load, if any.
+    subpage_id: Option<(PipelineId, SubpageId)>,
+    /// The current window size associated with this pipeline.
+    window_size: WindowSizeData,
+    /// Channel to the layout task associated with this pipeline.
+    layout_chan: LayoutChan,
+    /// The current viewport clipping rectangle applying to this pipelie, if any.
+    clip_rect: Option<Rect<f32>>,
+    /// The requested URL of the load.
+    url: Url,
+}
+
+impl InProgressLoad {
+    /// Create a new InProgressLoad object.
+    fn new(id: PipelineId,
+           subpage_id: Option<(PipelineId, SubpageId)>,
+           layout_chan: LayoutChan,
+           window_size: WindowSizeData,
+           url: Url) -> InProgressLoad {
+        InProgressLoad {
+            pipeline_id: id,
+            subpage_id: subpage_id,
+            layout_chan: layout_chan,
+            window_size: window_size,
+            clip_rect: None,
+            url: url,
+        }
+    }
+}
+
 #[derive(Copy)]
 pub enum TimerSource {
     FromWindow(PipelineId),
@@ -118,6 +167,8 @@ pub enum ScriptMsg {
     RunnableMsg(Box<Runnable+Send>),
     /// A DOM object's last pinned reference was removed (dispatched to all tasks).
     RefcountCleanup(TrustedReference),
+    /// The final network response for a page has arrived.
+    PageFetchComplete(PipelineId, Option<SubpageId>, LoadResponse),
 }
 
 /// A cloneable interface for communicating with an event loop.
@@ -175,11 +226,15 @@ impl Drop for StackRootTLS {
 /// FIXME: Rename to `Page`, following WebKit?
 pub struct ScriptTask {
     /// A handle to the information pertaining to page layout
-    page: DOMRefCell<Rc<Page>>,
+    page: DOMRefCell<Option<Rc<Page>>>,
+    /// A list of data pertaining to loads that have not yet received a network response
+    incomplete_loads: DOMRefCell<Vec<InProgressLoad>>,
     /// A handle to the image cache task.
     image_cache_task: ImageCacheTask,
     /// A handle to the resource task.
     resource_task: ResourceTask,
+    /// A handle to the storage task.
+    storage_task: StorageTask,
 
     /// The port on which the script task receives messages (load URL, exit, etc.)
     port: Receiver<ScriptMsg>,
@@ -242,7 +297,8 @@ impl<'a> Drop for ScriptMemoryFailsafe<'a> {
                 unsafe {
                     let page = owner.page.borrow_for_script_deallocation();
                     for page in page.iter() {
-                        *page.unsafe_mut_js_info() = None;
+                        let window = page.window().root();
+                        window.r().clear_js_context_for_script_deallocation();
                     }
                     *owner.js_context.borrow_for_script_deallocation() = None;
                 }
@@ -274,15 +330,14 @@ impl ScriptTaskFactory for ScriptTask {
                  storage_task: StorageTask,
                  image_cache_task: ImageCacheTask,
                  devtools_chan: Option<DevtoolsControlChan>,
-                 window_size: WindowSizeData)
+                 window_size: WindowSizeData,
+                 load_data: LoadData)
                  where C: ScriptListener + Send + 'static {
         let ConstellationChan(const_chan) = constellation_chan.clone();
         let (script_chan, script_port) = channel();
         let layout_chan = LayoutChan(layout_chan.sender());
         spawn_named_with_send_on_failure("ScriptTask", task_state::SCRIPT, move || {
-            let script_task = ScriptTask::new(id,
-                                              box compositor as Box<ScriptListener>,
-                                              layout_chan,
+            let script_task = ScriptTask::new(box compositor as Box<ScriptListener>,
                                               script_port,
                                               NonWorkerScriptChan(script_chan),
                                               control_chan,
@@ -291,9 +346,13 @@ impl ScriptTaskFactory for ScriptTask {
                                               resource_task,
                                               storage_task,
                                               image_cache_task,
-                                              devtools_chan,
-                                              window_size);
+                                              devtools_chan);
             let mut failsafe = ScriptMemoryFailsafe::new(&script_task);
+
+            let new_load = InProgressLoad::new(id, None, layout_chan, window_size,
+                                               load_data.url.clone());
+            script_task.start_page_load(new_load, load_data);
+
             script_task.start();
 
             // This must always be the very last operation performed before the task completes
@@ -312,9 +371,7 @@ unsafe extern "C" fn debug_gc_callback(_rt: *mut JSRuntime, status: JSGCStatus)
 
 impl ScriptTask {
     /// Creates a new script task.
-    pub fn new(id: PipelineId,
-               compositor: Box<ScriptListener+'static>,
-               layout_chan: LayoutChan,
+    pub fn new(compositor: Box<ScriptListener+'static>,
                port: Receiver<ScriptMsg>,
                chan: NonWorkerScriptChan,
                control_chan: ScriptControlChan,
@@ -323,8 +380,7 @@ impl ScriptTask {
                resource_task: ResourceTask,
                storage_task: StorageTask,
                img_cache_task: ImageCacheTask,
-               devtools_chan: Option<DevtoolsControlChan>,
-               window_size: WindowSizeData)
+               devtools_chan: Option<DevtoolsControlChan>)
                -> ScriptTask {
         let (js_runtime, js_context) = ScriptTask::new_rt_and_cx();
         let wrap_for_same_compartment = wrap_for_same_compartment as
@@ -348,19 +404,14 @@ impl ScriptTask {
                                       Some(pre_wrap));
         }
 
-        let page = Page::new(id, None, layout_chan, window_size,
-                             resource_task.clone(),
-                             storage_task,
-                             constellation_chan.clone(),
-                             js_context.clone(),
-                             devtools_chan.clone());
-
         let (devtools_sender, devtools_receiver) = channel();
         ScriptTask {
-            page: DOMRefCell::new(Rc::new(page)),
+            page: DOMRefCell::new(None),
+            incomplete_loads: DOMRefCell::new(vec!()),
 
             image_cache_task: img_cache_task,
             resource_task: resource_task,
+            storage_task: storage_task,
 
             port: port,
             chan: chan,
@@ -417,6 +468,11 @@ impl ScriptTask {
         (js_runtime, js_context)
     }
 
+    // Return the root page in the frame tree. Panics if it doesn't exist.
+    fn root_page(&self) -> Rc<Page> {
+        self.page.borrow().as_ref().unwrap().clone()
+    }
+
     pub fn get_cx(&self) -> *mut JSContext {
         (**self.js_context.borrow().as_ref().unwrap()).ptr
     }
@@ -439,17 +495,18 @@ impl ScriptTask {
         let mut resizes = vec!();
 
         {
-            let page = self.page.borrow_mut();
-            for page in page.iter() {
-                // Only process a resize if layout is idle.
-                let layout_join_port = page.layout_join_port.borrow();
-                if layout_join_port.is_none() {
-                    let mut resize_event = page.resize_event.get();
-                    match resize_event.take() {
-                        Some(size) => resizes.push((page.id, size)),
-                        None => ()
+            let page = self.page.borrow();
+            if let Some(page) = page.as_ref() {
+                for page in page.iter() {
+                    // Only process a resize if layout is idle.
+                    let window = page.window().root();
+                    if window.r().layout_is_idle() {
+                        let resize_event = window.r().steal_resize_event();
+                        match resize_event {
+                            Some(size) => resizes.push((window.r().pipeline(), size)),
+                            None => ()
+                        }
                     }
-                    page.resize_event.set(None);
                 }
             }
         }
@@ -502,17 +559,10 @@ impl ScriptTask {
                     self.handle_new_layout(new_layout_info);
                 }
                 MixedMessage::FromConstellation(ConstellationControlMsg::Resize(id, size)) => {
-                    let page = self.page.borrow_mut();
-                    let page = page.find(id).expect("resize sent to nonexistent pipeline");
-                    page.resize_event.set(Some(size));
+                    self.handle_resize(id, size);
                 }
                 MixedMessage::FromConstellation(ConstellationControlMsg::Viewport(id, rect)) => {
-                    let page = self.page.borrow_mut();
-                    let inner_page = page.find(id).expect("Page rect message sent to nonexistent pipeline");
-                    if inner_page.set_page_clip_rect_with_new_viewport(rect) {
-                        let page = get_page(&*page, id);
-                        self.force_reflow(&*page);
-                    }
+                    self.handle_viewport(id, rect);
                 }
                 _ => {
                     sequential.push(event);
@@ -555,8 +605,8 @@ impl ScriptTask {
         match msg {
             ConstellationControlMsg::AttachLayout(_) =>
                 panic!("should have handled AttachLayout already"),
-            ConstellationControlMsg::Load(id, parent, load_data) =>
-                self.load(id, parent, load_data),
+            ConstellationControlMsg::Activate(id) =>
+                self.handle_activate(id),
             ConstellationControlMsg::SendEvent(id, event) =>
                 self.handle_event(id, event),
             ConstellationControlMsg::ReflowComplete(id, reflow_id) =>
@@ -598,98 +648,144 @@ impl ScriptTask {
                 runnable.handler(),
             ScriptMsg::RefcountCleanup(addr) =>
                 LiveDOMReferences::cleanup(self.get_cx(), addr),
+            ScriptMsg::PageFetchComplete(id, subpage, response) =>
+                self.handle_page_fetch_complete(id, subpage, response),
         }
     }
 
     fn handle_msg_from_devtools(&self, msg: DevtoolScriptControlMsg) {
+        let page = self.root_page();
         match msg {
             DevtoolScriptControlMsg::EvaluateJS(id, s, reply) =>
-                devtools::handle_evaluate_js(&*self.page.borrow(), id, s, reply),
+                devtools::handle_evaluate_js(&page, id, s, reply),
             DevtoolScriptControlMsg::GetRootNode(id, reply) =>
-                devtools::handle_get_root_node(&*self.page.borrow(), id, reply),
+                devtools::handle_get_root_node(&page, id, reply),
             DevtoolScriptControlMsg::GetDocumentElement(id, reply) =>
-                devtools::handle_get_document_element(&*self.page.borrow(), id, reply),
+                devtools::handle_get_document_element(&page, id, reply),
             DevtoolScriptControlMsg::GetChildren(id, node_id, reply) =>
-                devtools::handle_get_children(&*self.page.borrow(), id, node_id, reply),
+                devtools::handle_get_children(&page, id, node_id, reply),
             DevtoolScriptControlMsg::GetLayout(id, node_id, reply) =>
-                devtools::handle_get_layout(&*self.page.borrow(), id, node_id, reply),
+                devtools::handle_get_layout(&page, id, node_id, reply),
             DevtoolScriptControlMsg::ModifyAttribute(id, node_id, modifications) =>
-                devtools::handle_modify_attribute(&*self.page.borrow(), id, node_id, modifications),
+                devtools::handle_modify_attribute(&page, id, node_id, modifications),
             DevtoolScriptControlMsg::WantsLiveNotifications(pipeline_id, to_send) =>
-                devtools::handle_wants_live_notifications(&*self.page.borrow(), pipeline_id, to_send),
+                devtools::handle_wants_live_notifications(&page, pipeline_id, to_send),
         }
     }
 
+    fn handle_resize(&self, id: PipelineId, size: WindowSizeData) {
+        let page = self.page.borrow();
+        if let Some(ref page) = page.as_ref() {
+            if let Some(ref page) = page.find(id) {
+                let window = page.window().root();
+                window.r().set_resize_event(size);
+                return;
+            }
+        }
+        let mut loads = self.incomplete_loads.borrow_mut();
+        if let Some(ref mut load) = loads.iter_mut().find(|load| load.pipeline_id == id) {
+            load.window_size = size;
+            return;
+        }
+        panic!("resize sent to nonexistent pipeline");
+    }
+
+    fn handle_viewport(&self, id: PipelineId, rect: Rect<f32>) {
+        let page = self.page.borrow();
+        if let Some(page) = page.as_ref() {
+            if let Some(ref inner_page) = page.find(id) {
+                let window = inner_page.window().root();
+                if window.r().set_page_clip_rect_with_new_viewport(rect) {
+                    let page = get_page(page, id);
+                    self.force_reflow(&*page);
+                }
+                return;
+            }
+        }
+        let mut loads = self.incomplete_loads.borrow_mut();
+        if let Some(ref mut load) = loads.iter_mut().find(|load| load.pipeline_id == id) {
+            load.clip_rect = Some(rect);
+            return;
+        }
+        panic!("Page rect message sent to nonexistent pipeline");
+     }
+
+    /// Handle a request to load a page in a new child frame of an existing page.
     fn handle_new_layout(&self, new_layout_info: NewLayoutInfo) {
         let NewLayoutInfo {
             old_pipeline_id,
             new_pipeline_id,
             subpage_id,
-            layout_chan
+            layout_chan,
+            load_data,
         } = new_layout_info;
 
-        let page = self.page.borrow_mut();
+        let page = self.root_page();
         let parent_page = page.find(old_pipeline_id).expect("ScriptTask: received a layout
             whose parent has a PipelineId which does not correspond to a pipeline in the script
             task's page tree. This is a bug.");
-        let new_page = {
-            let window_size = parent_page.window_size.get();
-            Page::new(new_pipeline_id, Some(subpage_id),
-                      LayoutChan(layout_chan.downcast_ref::<Sender<layout_interface::Msg>>().unwrap().clone()),
-                      window_size,
-                      parent_page.resource_task.clone(),
-                      parent_page.storage_task.clone(),
-                      self.constellation_chan.clone(),
-                      self.js_context.borrow().as_ref().unwrap().clone(),
-                      self.devtools_chan.clone())
-        };
-        parent_page.children.borrow_mut().push(Rc::new(new_page));
+
+        let parent_window = parent_page.window().root();
+        let chan = layout_chan.downcast_ref::<Sender<layout_interface::Msg>>().unwrap();
+        let layout_chan = LayoutChan(chan.clone());
+        // Kick off the fetch for the new resource.
+        let new_load = InProgressLoad::new(new_pipeline_id, Some((old_pipeline_id, subpage_id)),
+                                           layout_chan, parent_window.r().window_size(),
+                                           load_data.url.clone());
+        self.start_page_load(new_load, load_data);
     }
 
     /// Handles a timer that fired.
     fn handle_fire_timer_msg(&self, id: PipelineId, timer_id: TimerId) {
-        let page = self.page.borrow_mut();
+        let page = self.root_page();
         let page = page.find(id).expect("ScriptTask: received fire timer msg for a
             pipeline ID not associated with this script task. This is a bug.");
-        let frame = page.frame();
-        let window = frame.as_ref().unwrap().window.root();
+        let window = page.window().root();
         window.r().handle_fire_timer(timer_id);
     }
 
     /// Handles freeze message
     fn handle_freeze_msg(&self, id: PipelineId) {
-        let page = self.page.borrow_mut();
+        let page = self.root_page();
         let page = page.find(id).expect("ScriptTask: received freeze msg for a
                     pipeline ID not associated with this script task. This is a bug.");
-        let frame = page.frame();
-        let window = frame.as_ref().unwrap().window.root();
+        let window = page.window().root();
         window.r().freeze();
     }
 
     /// Handles thaw message
     fn handle_thaw_msg(&self, id: PipelineId) {
-        let page = self.page.borrow_mut();
+        let page = self.root_page();
         let page = page.find(id).expect("ScriptTask: received thaw msg for a
                             pipeline ID not associated with this script task. This is a bug.");
-        let frame = page.frame();
-        let window = frame.as_ref().unwrap().window.root();
+        let window = page.window().root();
         window.r().thaw();
     }
 
+    /// Handle a request to make a previously-created pipeline active.
+    //TODO: unsuspend JS and timers, etc. when we support such things.
+    fn handle_activate(&self, pipeline_id: PipelineId) {
+        // We should only get this message when moving in history, so all pages requested
+        // should exist.
+        let page = self.root_page().find(pipeline_id).unwrap();
+
+        let needed_reflow = page.set_reflow_status(false);
+        if needed_reflow {
+            self.force_reflow(&*page);
+        }
+    }
+
     /// Handles a notification that reflow completed.
     fn handle_reflow_complete_msg(&self, pipeline_id: PipelineId, reflow_id: uint) {
         debug!("Script: Reflow {:?} complete for {:?}", reflow_id, pipeline_id);
-        let page = self.page.borrow_mut();
+        let page = self.root_page();
         let page = page.find(pipeline_id).expect(
             "ScriptTask: received a load message for a layout channel that is not associated \
              with this script task. This is a bug.");
-        let last_reflow_id = page.last_reflow_id.get();
-        if last_reflow_id == reflow_id {
-            let mut layout_join_port = page.layout_join_port.borrow_mut();
-            *layout_join_port = None;
-        }
+        let window = page.window().root();
+        window.r().handle_reflow_complete_msg(reflow_id);
 
-        let doc = page.frame().as_ref().unwrap().document.root();
+        let doc = page.document().root();
         let html_element = doc.r().GetDocumentElement().root();
         let reftest_wait = html_element.r().map_or(false, |elem| elem.has_class(&Atom::from_slice("reftest-wait")));
 
@@ -700,14 +796,12 @@ impl ScriptTask {
 
     /// Window was resized, but this script was not active, so don't reflow yet
     fn handle_resize_inactive_msg(&self, id: PipelineId, new_size: WindowSizeData) {
-        let page = self.page.borrow_mut();
+        let page = self.root_page();
         let page = page.find(id).expect("Received resize message for PipelineId not associated
             with a page in the page tree. This is a bug.");
-        page.window_size.set(new_size);
-        match &mut *page.mut_url() {
-            &mut Some((_, ref mut needs_reflow)) => *needs_reflow = true,
-            &mut None => (),
-        }
+        let window = page.window().root();
+        window.r().set_window_size(new_size);
+        page.set_reflow_status(true);
     }
 
     /// We have gotten a window.close from script, which we pass on to the compositor.
@@ -724,182 +818,199 @@ impl ScriptTask {
         self.compositor.borrow_mut().close();
     }
 
+    /// We have received notification that the response associated with a load has completed.
+    /// Kick off the document and frame tree creation process using the result.
+    fn handle_page_fetch_complete(&self, id: PipelineId, subpage: Option<SubpageId>,
+                                  response: LoadResponse) {
+        // Any notification received should refer to an existing, in-progress load that is tracked.
+        let idx = self.incomplete_loads.borrow().iter().position(|&:load| {
+            load.pipeline_id == id && load.subpage_id.map(|sub| sub.1) == subpage
+        }).unwrap();
+        let load = self.incomplete_loads.borrow_mut().remove(idx);
+        self.load(response, load);
+    }
+
     /// Handles a request for the window title.
     fn handle_get_title_msg(&self, pipeline_id: PipelineId) {
-        get_page(&*self.page.borrow(), pipeline_id).send_title_to_compositor();
+        let page = get_page(&self.root_page(), pipeline_id);
+        let document = page.document().root();
+        document.r().send_title_to_compositor();
     }
 
     /// Handles a request to exit the script task and shut down layout.
     /// Returns true if the script task should shut down and false otherwise.
     fn handle_exit_pipeline_msg(&self, id: PipelineId, exit_type: PipelineExitType) -> bool {
         // If root is being exited, shut down all pages
-        let page = self.page.borrow_mut();
-        if page.id == id {
+        let page = self.root_page();
+        let window = page.window().root();
+        if window.r().pipeline() == id {
             debug!("shutting down layout for root page {:?}", id);
+            // To ensure the elements of the DOM tree remain usable (such as the window global),
+            // don't free the JS context until all interactions with it are finished.
+            shut_down_layout(&page, exit_type);
             *self.js_context.borrow_mut() = None;
-            shut_down_layout(&*page, (*self.js_runtime).ptr, exit_type);
             return true
         }
 
         // otherwise find just the matching page and exit all sub-pages
-        match page.find(id) {
-            Some(ref mut page) => {
-                shut_down_layout(&*page, (*self.js_runtime).ptr, exit_type);
-                page.remove(id);
-                false
-            }
-            // TODO(tkuehn): pipeline closing is currently duplicated across
-            // script and constellation, which can cause this to happen. Constellation
-            // needs to be smarter about exiting pipelines.
-            None => false,
+        if let Some(ref mut child_page) = page.remove(id) {
+            shut_down_layout(&*child_page, exit_type);
         }
-
+        return false;
     }
 
     /// The entry point to document loading. Defines bindings, sets up the window and document
     /// objects, parses HTML and CSS, and kicks off initial layout.
-    fn load(&self, pipeline_id: PipelineId,
-            parent: Option<(PipelineId, SubpageId)>, load_data: LoadData) {
-        let url = load_data.url.clone();
-        debug!("ScriptTask: loading {} on page {:?}", url.serialize(), pipeline_id);
-
-        let borrowed_page = self.page.borrow_mut();
-
-        let frame_element = parent.and_then(|(parent_id, subpage_id)| {
-            // In the case a parent id exists but the matching page
-            // cannot be found, this means the page exists in a different
-            // script task (due to origin) so it shouldn't be returned.
-            // TODO: window.parent will continue to return self in that
-            // case, which is wrong. We should be returning an object that
-            // denies access to most properties (per
-            // https://github.com/servo/servo/issues/3939#issuecomment-62287025).
-            borrowed_page.find(parent_id).and_then(|page| {
-                let doc = page.frame().as_ref().unwrap().document.root();
-                let doc: JSRef<Node> = NodeCast::from_ref(doc.r());
-
-                doc.traverse_preorder()
-                   .filter_map(HTMLIFrameElementCast::to_ref)
-                   .find(|node| node.subpage_id() == Some(subpage_id))
-                   .map(ElementCast::from_ref)
-                   .map(Temporary::from_rooted)
-            })
+    fn load(&self, response: LoadResponse, incomplete: InProgressLoad) {
+        let final_url = response.metadata.final_url.clone();
+        debug!("ScriptTask: loading {} on page {:?}", incomplete.url.serialize(), incomplete.pipeline_id);
+
+        // We should either be initializing a root page or loading a child page of an
+        // existing one.
+        let root_page_exists = self.page.borrow().is_some();
+        assert!(incomplete.subpage_id.is_none() || root_page_exists);
+
+        let frame_element = incomplete.subpage_id.and_then(|(parent_id, subpage_id)| {
+            let borrowed_page = self.root_page();
+          // In the case a parent id exists but the matching page
+          // cannot be found, this means the page exists in a different
+          // script task (due to origin) so it shouldn't be returned.
+          // TODO: window.parent will continue to return self in that
+          // case, which is wrong. We should be returning an object that
+          // denies access to most properties (per
+          // https://github.com/servo/servo/issues/3939#issuecomment-62287025).
+          borrowed_page.find(parent_id).and_then(|page| {
+            let doc = page.document().root();
+            let doc: JSRef<Node> = NodeCast::from_ref(doc.r());
+
+            doc.traverse_preorder()
+               .filter_map(HTMLIFrameElementCast::to_ref)
+               .find(|node| node.subpage_id() == Some(subpage_id))
+               .map(ElementCast::from_ref)
+               .map(Temporary::from_rooted)
+          })
         }).root();
 
-        let page = borrowed_page.find(pipeline_id).expect("ScriptTask: received a load
-            message for a layout channel that is not associated with this script task. This
-            is a bug.");
+        self.compositor.borrow_mut().set_ready_state(incomplete.pipeline_id, Loading);
 
-        // Are we reloading?
-        let reloading = match *page.url() {
-            Some((ref loaded, _)) => *loaded == url,
-            _ => false,
-        };
-        if reloading {
-            // Pull out the `needs_reflow` flag explicitly because `reflow` can ask for the page's
-            // URL, and we can't be holding a borrow on that URL (#4402).
-            let needed_reflow = match &mut *page.mut_url() {
-                &mut Some((_, ref mut needs_reflow)) => replace(needs_reflow, false),
-                _ => panic!("can't reload a page with no URL!")
-            };
-            if needed_reflow {
-                self.force_reflow(&*page);
-            }
-            return
-        }
-        let last_url = replace(&mut *page.mut_url(), None).map(|(last_url, _)| last_url);
-
-        let is_javascript = url.scheme.as_slice() == "javascript";
-
-        self.compositor.borrow_mut().set_ready_state(pipeline_id, Loading);
-
-        let (mut parser_input, final_url, last_modified) = if !is_javascript {
-            // Wait for the LoadResponse so that the parser knows the final URL.
-            let (input_chan, input_port) = channel();
-            self.resource_task.send(ControlMsg::Load(NetLoadData {
-                url: url,
-                method: load_data.method,
-                headers: Headers::new(),
-                preserved_headers: load_data.headers,
-                data: load_data.data,
-                cors: None,
-                consumer: input_chan,
-            })).unwrap();
+        let cx = self.js_context.borrow();
+        let cx = cx.as_ref().unwrap();
 
-            let load_response = input_port.recv().unwrap();
+        // Create a new frame tree entry.
+        let page = Rc::new(Page::new(incomplete.pipeline_id, incomplete.subpage_id.map(|p| p.1),
+                                     final_url.clone()));
+        if !root_page_exists {
+            // We have a new root frame tree.
+            *self.page.borrow_mut() = Some(page.clone());
+        } else if let Some((parent, _)) = incomplete.subpage_id {
+            // We have a new child frame.
+            let parent_page = self.root_page();
+            parent_page.find(parent).expect("received load for subpage with missing parent");
+            parent_page.children.borrow_mut().push(page.clone());
+        }
 
-            let last_modified = load_response.metadata.headers.as_ref().and_then(|headers| {
-                headers.get().map(|&LastModified(ref tm)| tm.clone())
-            });
+        enum PageToRemove {
+            Root,
+            Child(PipelineId),
+        }
+        struct AutoPageRemover<'a> {
+            page: PageToRemove,
+            script_task: &'a ScriptTask,
+            neutered: bool,
+        }
+        impl<'a> AutoPageRemover<'a> {
+            fn new(script_task: &'a ScriptTask, page: PageToRemove) -> AutoPageRemover<'a> {
+                AutoPageRemover {
+                    page: page,
+                    script_task: script_task,
+                    neutered: false,
+                }
+            }
 
-            let final_url = load_response.metadata.final_url.clone();
+            fn neuter(&mut self) {
+                self.neutered = true;
+            }
+        }
+        #[unsafe_destructor]
+        impl<'a> Drop for AutoPageRemover<'a> {
+            fn drop(&mut self) {
+                if !self.neutered {
+                    match self.page {
+                        PageToRemove::Root => *self.script_task.page.borrow_mut() = None,
+                        PageToRemove::Child(id) => {
+                            let _ = self.script_task.root_page().remove(id);
+                        }
+                    }
+                }
+            }
+        }
 
-            (Some(HTMLInput::InputUrl(load_response)), final_url, last_modified)
+        let page_to_remove = if !root_page_exists {
+            PageToRemove::Root
         } else {
-            let doc_url = last_url.unwrap_or_else(|| {
-                Url::parse("about:blank").unwrap()
-            });
-            (None, doc_url, None)
+            PageToRemove::Child(incomplete.pipeline_id)
         };
+        let mut page_remover = AutoPageRemover::new(self, page_to_remove);
 
-        // Store the final URL before we start parsing, so that DOM routines
-        // (e.g. HTMLImageElement::update_image) can resolve relative URLs
-        // correctly.
-        {
-            *page.mut_url() = Some((final_url.clone(), true));
-        }
-
-        let cx = self.js_context.borrow();
-        let cx = cx.as_ref().unwrap();
         // Create the window and document objects.
-        let window = Window::new(cx.ptr,
+        let window = Window::new(cx.clone(),
                                  page.clone(),
                                  self.chan.clone(),
                                  self.control_chan.clone(),
                                  self.compositor.borrow_mut().dup(),
-                                 self.image_cache_task.clone()).root();
+                                 self.image_cache_task.clone(),
+                                 self.resource_task.clone(),
+                                 self.storage_task.clone(),
+                                 self.devtools_chan.clone(),
+                                 self.constellation_chan.clone(),
+                                 incomplete.layout_chan,
+                                 incomplete.pipeline_id,
+                                 incomplete.subpage_id.map(|s| s.1),
+                                 incomplete.window_size).root();
 
         let document = Document::new(window.r(), Some(final_url.clone()),
                                      IsHTMLDocument::HTMLDocument, None,
                                      DocumentSource::FromParser).root();
+
+        window.r().init_browser_context(document.r(), frame_element.r());
+
+        let last_modified = response.metadata.headers.as_ref().and_then(|headers| {
+            headers.get().map(|&LastModified(ref tm)| tm.clone())
+        });
         if let Some(tm) = last_modified {
             document.r().set_last_modified(dom_last_modified(&tm));
         }
-        window.r().init_browser_context(document.r(), frame_element.r());
 
+        // Create the root frame
+        page.set_frame(Some(Frame {
+            document: JS::from_rooted(document.r()),
+            window: JS::from_rooted(window.r()),
+        }));
 
-        {
-            // Create the root frame.
-            let mut frame = page.mut_frame();
-            *frame = Some(Frame {
-                document: JS::from_rooted(document.r()),
-                window: JS::from_rooted(window.r()),
-            });
-        }
-
-        if is_javascript {
-            let evalstr = load_data.url.non_relative_scheme_data().unwrap();
+        let is_javascript = incomplete.url.scheme.as_slice() == "javascript";
+        let parse_input = if is_javascript {
+            let evalstr = incomplete.url.non_relative_scheme_data().unwrap();
             let jsval = window.r().evaluate_js_on_global_with_result(evalstr);
             let strval = FromJSValConvertible::from_jsval(self.get_cx(), jsval,
                                                           StringificationBehavior::Empty);
-            parser_input = Some(HTMLInput::InputString(strval.unwrap_or("".to_owned())));
+            HTMLInput::InputString(strval.unwrap_or("".to_owned()))
+        } else {
+            HTMLInput::InputUrl(response)
         };
 
-        parse_html(document.r(), parser_input.unwrap(), &final_url);
+        parse_html(document.r(), parse_input, &final_url);
 
         document.r().set_ready_state(DocumentReadyState::Interactive);
-        self.compositor.borrow_mut().set_ready_state(pipeline_id, PerformingLayout);
+        self.compositor.borrow_mut().set_ready_state(incomplete.pipeline_id, PerformingLayout);
 
         // Kick off the initial reflow of the page.
         debug!("kicking off initial reflow of {:?}", final_url);
         document.r().content_changed(NodeCast::from_ref(document.r()),
                                      NodeDamage::OtherNodeDamage);
-        window.r().flush_layout(ReflowGoal::ForDisplay, ReflowQueryType::NoQuery);
+        window.r().reflow(ReflowGoal::ForDisplay, ReflowQueryType::NoQuery);
 
-        {
-            // No more reflow required
-            let mut page_url = page.mut_url();
-            *page_url = Some((final_url.clone(), false));
-        }
+        // No more reflow required
+        page.set_reflow_status(false);
 
         // https://html.spec.whatwg.org/multipage/#the-end step 4
         let addr: Trusted<Document> = Trusted::new(self.get_cx(), document.r(), self.chan.clone());
@@ -914,7 +1025,7 @@ impl ScriptTask {
         let handler = Box::new(DocumentProgressHandler::new(addr, DocumentProgressTask::Load));
         self.chan.send(ScriptMsg::RunnableMsg(handler)).unwrap();
 
-        *page.fragment_name.borrow_mut() = final_url.fragment.clone();
+        window.r().set_fragment_name(final_url.fragment.clone());
 
         let ConstellationChan(ref chan) = self.constellation_chan;
         chan.send(ConstellationMsg::LoadComplete).unwrap();
@@ -927,11 +1038,13 @@ impl ScriptTask {
                     title: document.r().Title(),
                     url: final_url
                 };
-                chan.send(DevtoolsControlMsg::NewGlobal(pipeline_id,
+                chan.send(DevtoolsControlMsg::NewGlobal(incomplete.pipeline_id,
                                                         self.devtools_sender.clone(),
                                                         page_info)).unwrap();
             }
         }
+
+        page_remover.neuter();
     }
 
     fn scroll_fragment_point(&self, pipeline_id: PipelineId, node: JSRef<Element>) {
@@ -948,11 +1061,10 @@ impl ScriptTask {
 
     /// Reflows non-incrementally.
     fn force_reflow(&self, page: &Page) {
-        page.dirty_all_nodes();
-        page.reflow(ReflowGoal::ForDisplay,
-                    self.control_chan.clone(),
-                    &mut **self.compositor.borrow_mut(),
-                    ReflowQueryType::NoQuery);
+        let document = page.document().root();
+        document.r().dirty_all_nodes();
+        let window = window_from_node(document.r()).root();
+        window.r().reflow(ReflowGoal::ForDisplay, ReflowQueryType::NoQuery);
     }
 
     /// This is the main entry point for receiving and dispatching DOM events.
@@ -978,9 +1090,8 @@ impl ScriptTask {
                 for node in nodes.iter() {
                     let node_to_dirty = node::from_untrusted_node_address(self.js_runtime.ptr,
                                                                           *node).root();
-                    let page = get_page(&*self.page.borrow(), pipeline_id);
-                    let frame = page.frame();
-                    let document = frame.as_ref().unwrap().document.root();
+                    let page = get_page(&self.root_page(), pipeline_id);
+                    let document = page.document().root();
                     document.r().content_changed(node_to_dirty.r(),
                                                  NodeDamage::OtherNodeDamage);
                 }
@@ -989,18 +1100,16 @@ impl ScriptTask {
             }
 
             ClickEvent(_button, point) => {
-                let page = get_page(&*self.page.borrow(), pipeline_id);
-                let frame = page.frame();
-                let document = frame.as_ref().unwrap().document.root();
+                let page = get_page(&self.root_page(), pipeline_id);
+                let document = page.document().root();
                 document.r().handle_click_event(self.js_runtime.ptr, _button, point);
             }
 
             MouseDownEvent(..) => {}
             MouseUpEvent(..) => {}
             MouseMoveEvent(point) => {
-                let page = get_page(&*self.page.borrow(), pipeline_id);
-                let frame = page.frame();
-                let document = frame.as_ref().unwrap().document.root();
+                let page = get_page(&self.root_page(), pipeline_id);
+                let document = page.document().root();
                 let mouse_over_targets = &mut *self.mouse_over_targets.borrow_mut();
 
                 if document.r().handle_mouse_move_event(self.js_runtime.ptr, point, mouse_over_targets) {
@@ -1009,9 +1118,8 @@ impl ScriptTask {
             }
 
             KeyEvent(key, state, modifiers) => {
-                let page = get_page(&*self.page.borrow(), pipeline_id);
-                let frame = page.frame();
-                let document = frame.as_ref().unwrap().document.root();
+                let page = get_page(&self.root_page(), pipeline_id);
+                let document = page.document().root();
                 document.r().dispatch_key_event(
                     key, state, modifiers, &mut *self.compositor.borrow_mut());
             }
@@ -1028,8 +1136,9 @@ impl ScriptTask {
     /// The entry point for content to notify that a fragment url has been requested
     /// for the given pipeline.
     fn trigger_fragment(&self, pipeline_id: PipelineId, fragment: String) {
-        let page = get_page(&*self.page.borrow(), pipeline_id);
-        match page.find_fragment_node(fragment).root() {
+        let page = get_page(&self.root_page(), pipeline_id);
+        let document = page.document().root();
+        match document.r().find_fragment_node(fragment).root() {
             Some(node) => {
                 self.scroll_fragment_point(pipeline_id, node.r());
             }
@@ -1039,87 +1148,97 @@ impl ScriptTask {
 
 
     fn handle_resize_event(&self, pipeline_id: PipelineId, new_size: WindowSizeData) {
-        let window = {
-            let page = get_page(&*self.page.borrow(), pipeline_id);
-            page.window_size.set(new_size);
-
-            let frame = page.frame();
-            if frame.is_some() {
-                self.force_reflow(&*page);
-            }
-
-            let fragment_node =
-                page.fragment_name
-                    .borrow_mut()
-                    .take()
-                    .and_then(|name| page.find_fragment_node(name))
-                    .root();
-            match fragment_node {
-                Some(node) => self.scroll_fragment_point(pipeline_id, node.r()),
-                None => {}
-            }
+        let page = get_page(&self.root_page(), pipeline_id);
+        let window = page.window().root();
+        window.r().set_window_size(new_size);
+        self.force_reflow(&*page);
+
+        let document = page.document().root();
+        let fragment_node = window.r().steal_fragment_name()
+                                      .and_then(|name| document.r().find_fragment_node(name))
+                                      .root();
+        match fragment_node {
+            Some(node) => self.scroll_fragment_point(pipeline_id, node.r()),
+            None => {}
+        }
 
-            frame.as_ref().map(|frame| Temporary::new(frame.window.clone()))
-        };
+        // http://dev.w3.org/csswg/cssom-view/#resizing-viewports
+        // https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#event-type-resize
+        let uievent = UIEvent::new(window.r(),
+                                   "resize".to_owned(), false,
+                                   false, Some(window.r()),
+                                   0i32).root();
+        let event: JSRef<Event> = EventCast::from_ref(uievent.r());
 
-        match window.root() {
-            Some(window) => {
-                // http://dev.w3.org/csswg/cssom-view/#resizing-viewports
-                // https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#event-type-resize
-                let uievent = UIEvent::new(window.r(),
-                                           "resize".to_owned(), false,
-                                           false, Some(window.r()),
-                                           0i32).root();
-                let event: JSRef<Event> = EventCast::from_ref(uievent.r());
-
-                let wintarget: JSRef<EventTarget> = EventTargetCast::from_ref(window.r());
-                event.fire(wintarget);
-            }
-            None => ()
-        }
+        let wintarget: JSRef<EventTarget> = EventTargetCast::from_ref(window.r());
+        event.fire(wintarget);
     }
 
     fn handle_reflow_event(&self, pipeline_id: PipelineId) {
         debug!("script got reflow event");
-        let page = get_page(&*self.page.borrow(), pipeline_id);
-        let frame = page.frame();
-        if frame.is_some() {
-            self.force_reflow(&*page);
-        }
+        let page = get_page(&self.root_page(), pipeline_id);
+        self.force_reflow(&*page);
+    }
+
+    /// Initiate a non-blocking fetch for a specified resource. Stores the InProgressLoad
+    /// argument until a notification is received that the fetch is complete.
+    fn start_page_load(&self, incomplete: InProgressLoad, mut load_data: LoadData) {
+        let id = incomplete.pipeline_id.clone();
+        let subpage = incomplete.subpage_id.clone().map(|p| p.1);
+
+        let script_chan = self.chan.clone();
+        let resource_task = self.resource_task.clone();
+
+        spawn_named(format!("fetch for {:?}", load_data.url), move || {
+            if load_data.url.scheme.as_slice() == "javascript" {
+                load_data.url = Url::parse("about:blank").unwrap();
+            }
+
+            let (input_chan, input_port) = channel();
+            resource_task.send(ControlMsg::Load(NetLoadData {
+                url: load_data.url,
+                method: load_data.method,
+                headers: Headers::new(),
+                preserved_headers: load_data.headers,
+                data: load_data.data,
+                cors: None,
+                consumer: input_chan,
+            })).unwrap();
+
+            let load_response = input_port.recv().unwrap();
+            script_chan.send(ScriptMsg::PageFetchComplete(id, subpage, load_response)).unwrap();
+        });
+
+        self.incomplete_loads.borrow_mut().push(incomplete);
     }
 }
 
 /// Shuts down layout for the given page tree.
-fn shut_down_layout(page_tree: &Rc<Page>, rt: *mut JSRuntime, exit_type: PipelineExitType) {
+fn shut_down_layout(page_tree: &Rc<Page>, exit_type: PipelineExitType) {
+    let mut channels = vec!();
+
     for page in page_tree.iter() {
         // Tell the layout task to begin shutting down, and wait until it
         // processed this message.
         let (response_chan, response_port) = channel();
-        let LayoutChan(ref chan) = page.layout_chan;
+        let window = page.window().root();
+        let LayoutChan(chan) = window.r().layout_chan();
         if chan.send(layout_interface::Msg::PrepareToExit(response_chan)).is_ok() {
-          response_port.recv().unwrap();
+            channels.push(chan);
+            response_port.recv().unwrap();
         }
     }
 
-    // Remove our references to the DOM objects in this page tree.
-    for page in page_tree.iter() {
-        *page.mut_frame() = None;
-    }
-
-    // Drop our references to the JSContext, potentially triggering a GC.
+    // Drop our references to the JSContext and DOM objects.
     for page in page_tree.iter() {
-        *page.mut_js_info() = None;
-    }
-
-    // Force a GC to make sure that our DOM reflectors are released before we tell
-    // layout to exit.
-    unsafe {
-        JS_GC(rt);
+        let window = page.window().root();
+        window.r().clear_js_context();
+        // Sever the connection between the global and the DOM tree
+        page.set_frame(None);
     }
 
     // Destroy the layout task. If there were node leaks, layout will now crash safely.
-    for page in page_tree.iter() {
-        let LayoutChan(ref chan) = page.layout_chan;
+    for chan in channels.into_iter() {
         chan.send(layout_interface::Msg::ExitNow(exit_type)).ok();
     }
 }