diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 52c617fb0bd..1614f28b741 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -754,10 +754,15 @@ impl<Window: WindowMethods> IOCompositor<Window> {
 
         let initial_viewport = self.window_rect.size.to_f32() / dppx;
 
-        let msg = ConstellationMsg::WindowSize(WindowSizeData {
+        let data = WindowSizeData {
             device_pixel_ratio: dppx,
             initial_viewport: initial_viewport,
-        }, size_type);
+        };
+        let top_level_browsing_context_id = match self.root_pipeline {
+            Some(ref pipeline) => pipeline.top_level_browsing_context_id,
+            None => return warn!("Window resize without root pipeline."),
+        };
+        let msg = ConstellationMsg::WindowSize(top_level_browsing_context_id, data, size_type);
 
         if let Err(e) = self.constellation_chan.send(msg) {
             warn!("Sending window resize to constellation failed ({}).", e);
@@ -866,7 +871,11 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             }
 
             WindowEvent::Reload => {
-                let msg = ConstellationMsg::Reload;
+                let top_level_browsing_context_id = match self.root_pipeline {
+                    Some(ref pipeline) => pipeline.top_level_browsing_context_id,
+                    None => return warn!("Window reload without root pipeline."),
+                };
+                let msg = ConstellationMsg::Reload(top_level_browsing_context_id);
                 if let Err(e) = self.constellation_chan.send(msg) {
                     warn!("Sending reload to constellation failed ({}).", e);
                 }
@@ -1338,7 +1347,11 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             windowing::WindowNavigateMsg::Forward => TraversalDirection::Forward(1),
             windowing::WindowNavigateMsg::Back => TraversalDirection::Back(1),
         };
-        let msg = ConstellationMsg::TraverseHistory(None, direction);
+        let top_level_browsing_context_id = match self.root_pipeline {
+            Some(ref pipeline) => pipeline.top_level_browsing_context_id,
+            None => return warn!("Sending navigation to constellation with no root pipeline."),
+        };
+        let msg = ConstellationMsg::TraverseHistory(top_level_browsing_context_id, direction);
         if let Err(e) = self.constellation_chan.send(msg) {
             warn!("Sending navigation to constellation failed ({}).", e);
         }
diff --git a/components/compositing/lib.rs b/components/compositing/lib.rs
index 9b3fb8d9cda..e4307ff04cc 100644
--- a/components/compositing/lib.rs
+++ b/components/compositing/lib.rs
@@ -29,6 +29,7 @@ pub use compositor::IOCompositor;
 use euclid::size::TypedSize2D;
 use ipc_channel::ipc::IpcSender;
 use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::TopLevelBrowsingContextId;
 use script_traits::{ConstellationControlMsg, LayoutControlMsg};
 use style_traits::CSSPixel;
 
@@ -48,6 +49,7 @@ pub struct SendableFrameTree {
 #[derive(Clone)]
 pub struct CompositionPipeline {
     pub id: PipelineId,
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
     pub script_chan: IpcSender<ConstellationControlMsg>,
     pub layout_chan: IpcSender<LayoutControlMsg>,
 }
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 7567b3d12cd..51d22e1129c 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -863,6 +863,13 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 debug!("constellation got get root pipeline message");
                 self.handle_get_pipeline(browsing_context_id, resp_chan);
             }
+            FromCompositorMsg::GetFocusTopLevelBrowsingContext(resp_chan) => {
+                debug!("constellation got get focus browsing context message");
+                let focus_browsing_context = self.focus_pipeline_id
+                    .and_then(|pipeline_id| self.pipelines.get(&pipeline_id))
+                    .map(|pipeline| pipeline.top_level_browsing_context_id);
+                let _ = resp_chan.send(focus_browsing_context);
+            }
             FromCompositorMsg::GetPipelineTitle(pipeline_id) => {
                 debug!("constellation got get-pipeline-title message");
                 self.handle_get_pipeline_title_msg(pipeline_id);
@@ -896,13 +903,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 self.handle_init_load(url);
             }
             // Handle a forward or back request
-            FromCompositorMsg::TraverseHistory(pipeline_id, direction) => {
+            // Handle a forward or back request
+            FromCompositorMsg::TraverseHistory(top_level_browsing_context_id, direction) => {
                 debug!("constellation got traverse history message from compositor");
-                self.handle_traverse_history_msg(pipeline_id, direction);
+                self.handle_traverse_history_msg(top_level_browsing_context_id, direction);
             }
-            FromCompositorMsg::WindowSize(new_size, size_type) => {
+            FromCompositorMsg::WindowSize(top_level_browsing_context_id, new_size, size_type) => {
                 debug!("constellation got window resize message");
-                self.handle_window_size_msg(new_size, size_type);
+                self.handle_window_size_msg(top_level_browsing_context_id, new_size, size_type);
             }
             FromCompositorMsg::TickAnimation(pipeline_id, tick_type) => {
                 self.handle_tick_animation(pipeline_id, tick_type)
@@ -911,9 +919,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 debug!("constellation got webdriver command message");
                 self.handle_webdriver_msg(command);
             }
-            FromCompositorMsg::Reload => {
+            FromCompositorMsg::Reload(top_level_browsing_context_id) => {
                 debug!("constellation got reload message");
-                self.handle_reload_msg();
+                self.handle_reload_msg(top_level_browsing_context_id);
             }
             FromCompositorMsg::LogEntry(top_level_browsing_context_id, thread_name, entry) => {
                 self.handle_log_entry(top_level_browsing_context_id, thread_name, entry);
@@ -963,14 +971,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 self.handle_load_complete_msg(pipeline_id)
             }
             // Handle a forward or back request
-            FromScriptMsg::TraverseHistory(pipeline_id, direction) => {
+            FromScriptMsg::TraverseHistory(top_level_browsing_context_id, direction) => {
                 debug!("constellation got traverse history message from script");
-                self.handle_traverse_history_msg(pipeline_id, direction);
+                self.handle_traverse_history_msg(top_level_browsing_context_id, direction);
             }
             // Handle a joint session history length request.
-            FromScriptMsg::JointSessionHistoryLength(pipeline_id, sender) => {
+            FromScriptMsg::JointSessionHistoryLength(top_level_browsing_context_id, sender) => {
                 debug!("constellation got joint session history length message from script");
-                self.handle_joint_session_history_length(pipeline_id, sender);
+                self.handle_joint_session_history_length(top_level_browsing_context_id, sender);
             }
             // Notification that the new document is ready to become active
             FromScriptMsg::ActivateDocument(pipeline_id) => {
@@ -1392,6 +1400,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         let window_size = self.window_size.initial_viewport;
         let root_pipeline_id = PipelineId::new();
         let root_browsing_context_id = self.root_browsing_context_id;
+        self.focus_pipeline_id = Some(root_pipeline_id);
         let load_data = LoadData::new(url.clone(), None, None, None);
         let sandbox = IFrameSandboxState::IFrameUnsandboxed;
         self.new_pipeline(root_pipeline_id,
@@ -1530,6 +1539,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
 
             Pipeline::new(new_pipeline_id,
                           browsing_context_id,
+                          top_level_browsing_context_id,
                           Some((parent_pipeline_id, frame_type)),
                           script_sender,
                           layout_sender,
@@ -1747,14 +1757,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn handle_traverse_history_msg(&mut self,
-                                   pipeline_id: Option<PipelineId>,
-                                   direction: TraversalDirection) {
-        let top_level_browsing_context_id = pipeline_id
-            .and_then(|pipeline_id| self.pipelines.get(&pipeline_id))
-            .and_then(|pipeline| self.browsing_contexts.get(&pipeline.browsing_context_id))
-            .map(|browsing_context| browsing_context.top_level_id)
-            .unwrap_or(self.root_browsing_context_id);
-
+                                   top_level_browsing_context_id: TopLevelBrowsingContextId,
+                                   direction: TraversalDirection)
+    {
         let mut size = 0;
         let mut table = HashMap::new();
 
@@ -1784,12 +1789,10 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
     }
 
-    fn handle_joint_session_history_length(&self, pipeline_id: PipelineId, sender: IpcSender<u32>) {
-        let top_level_browsing_context_id = self.pipelines.get(&pipeline_id)
-            .and_then(|pipeline| self.browsing_contexts.get(&pipeline.browsing_context_id))
-            .map(|browsing_context| browsing_context.top_level_id)
-            .unwrap_or(self.root_browsing_context_id);
-
+    fn handle_joint_session_history_length(&self,
+                                           top_level_browsing_context_id: TopLevelBrowsingContextId,
+                                           sender: IpcSender<u32>)
+    {
         // Initialize length at 1 to count for the current active entry
         let mut length = 1;
         for browsing_context in self.all_browsing_contexts_iter(top_level_browsing_context_id) {
@@ -1827,21 +1830,19 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         }
     }
 
-    fn handle_reload_msg(&mut self) {
-        // Send Reload constellation msg to root script channel.
-        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
-        let root_pipeline_id = self.browsing_contexts.get(&root_browsing_context_id)
-            .map(|root_browsing_context| root_browsing_context.pipeline_id);
-
-        if let Some(pipeline_id) = root_pipeline_id {
-            let msg = ConstellationControlMsg::Reload(pipeline_id);
-            let result = match self.pipelines.get(&pipeline_id) {
-                Some(pipeline) => pipeline.event_loop.send(msg),
-                None => return debug!("Pipeline {:?} got reload event after closure.", pipeline_id),
-            };
-            if let Err(e) = result {
-                self.handle_send_error(pipeline_id, e);
-            }
+    fn handle_reload_msg(&mut self, top_level_browsing_context_id: TopLevelBrowsingContextId) {
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+        let pipeline_id = match self.browsing_contexts.get(&browsing_context_id) {
+            Some(browsing_context) => browsing_context.pipeline_id,
+            None => return warn!("Browsing context {} got reload event after closure.", browsing_context_id),
+        };
+        let msg = ConstellationControlMsg::Reload(pipeline_id);
+        let result = match self.pipelines.get(&pipeline_id) {
+            None => return warn!("Pipeline {} got reload event after closure.", pipeline_id),
+            Some(pipeline) => pipeline.event_loop.send(msg),
+        };
+        if let Err(e) = result {
+            self.handle_send_error(pipeline_id, e);
         }
     }
 
@@ -1894,10 +1895,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn handle_get_pipeline(&mut self,
-                           browsing_context_id: Option<BrowsingContextId>,
+                           browsing_context_id: BrowsingContextId,
                            resp_chan: IpcSender<Option<PipelineId>>) {
-        let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
-        let browsing_context_id = browsing_context_id.unwrap_or(root_browsing_context_id);
         let current_pipeline_id = self.browsing_contexts.get(&browsing_context_id)
             .map(|browsing_context| browsing_context.pipeline_id);
         let pipeline_id_loaded = self.pending_changes.iter().rev()
@@ -2033,17 +2032,22 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 self.webdriver.resize_channel = Some(reply);
                 self.compositor_proxy.send(ToCompositorMsg::ResizeTo(size));
             },
-            WebDriverCommandMsg::LoadUrl(pipeline_id, load_data, reply) => {
-                self.load_url_for_webdriver(pipeline_id, load_data, reply, false);
+            WebDriverCommandMsg::LoadUrl(top_level_browsing_context_id, load_data, reply) => {
+                self.load_url_for_webdriver(top_level_browsing_context_id, load_data, reply, false);
             },
-            WebDriverCommandMsg::Refresh(pipeline_id, reply) => {
-                let load_data = match self.pipelines.get(&pipeline_id) {
-                    Some(pipeline) => LoadData::new(pipeline.url.clone(), None, None, None),
-                    None => return warn!("Pipeline {:?} Refresh after closure.", pipeline_id),
+            WebDriverCommandMsg::Refresh(top_level_browsing_context_id, reply) => {
+                let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+                let load_data = match self.browsing_contexts.get(&browsing_context_id) {
+                    Some(browsing_context) => browsing_context.load_data.clone(),
+                    None => return warn!("Browsing context {} Refresh after closure.", browsing_context_id),
                 };
-                self.load_url_for_webdriver(pipeline_id, load_data, reply, true);
+                self.load_url_for_webdriver(top_level_browsing_context_id, load_data, reply, true);
             }
-            WebDriverCommandMsg::ScriptCommand(pipeline_id, cmd) => {
+            WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd) => {
+                let pipeline_id = match self.browsing_contexts.get(&browsing_context_id) {
+                    Some(browsing_context) => browsing_context.pipeline_id,
+                    None => return warn!("Browsing context {} ScriptCommand after closure.", browsing_context_id),
+                };
                 let control_msg = ConstellationControlMsg::WebDriverScriptCommand(pipeline_id, cmd);
                 let result = match self.pipelines.get(&pipeline_id) {
                     Some(pipeline) => pipeline.event_loop.send(control_msg),
@@ -2053,10 +2057,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                     self.handle_send_error(pipeline_id, e);
                 }
             },
-            WebDriverCommandMsg::SendKeys(pipeline_id, cmd) => {
+            WebDriverCommandMsg::SendKeys(browsing_context_id, cmd) => {
+                let pipeline_id = match self.browsing_contexts.get(&browsing_context_id) {
+                    Some(browsing_context) => browsing_context.pipeline_id,
+                    None => return warn!("Browsing context {} SendKeys after closure.", browsing_context_id),
+                };
                 let event_loop = match self.pipelines.get(&pipeline_id) {
                     Some(pipeline) => pipeline.event_loop.clone(),
-                    None => return warn!("Pipeline {:?} SendKeys after closure.", pipeline_id),
+                    None => return warn!("Pipeline {} SendKeys after closure.", pipeline_id),
                 };
                 for (key, mods, state) in cmd {
                     let event = CompositorEvent::KeyEvent(None, key, state, mods);
@@ -2066,17 +2074,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                     }
                 }
             },
-            WebDriverCommandMsg::TakeScreenshot(pipeline_id, reply) => {
-                let root_browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
-                let current_pipeline_id = self.browsing_contexts.get(&root_browsing_context_id)
-                    .map(|root_browsing_context| root_browsing_context.pipeline_id);
-                if Some(pipeline_id) == current_pipeline_id {
-                    self.compositor_proxy.send(ToCompositorMsg::CreatePng(reply));
-                } else {
-                    if let Err(e) = reply.send(None) {
-                        warn!("Screenshot reply failed ({})", e);
-                    }
-                }
+            WebDriverCommandMsg::TakeScreenshot(_, reply) => {
+                self.compositor_proxy.send(ToCompositorMsg::CreatePng(reply));
             },
         }
     }
@@ -2267,13 +2266,18 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     fn load_url_for_webdriver(&mut self,
-                              pipeline_id: PipelineId,
+                              top_level_browsing_context_id: TopLevelBrowsingContextId,
                               load_data: LoadData,
                               reply: IpcSender<webdriver_msg::LoadStatus>,
-                              replace: bool) {
-        let new_pipeline_id = self.load_url(pipeline_id, load_data, replace);
-        if let Some(id) = new_pipeline_id {
-            self.webdriver.load_channel = Some((id, reply));
+                              replace: bool)
+    {
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+        let pipeline_id = match self.browsing_contexts.get(&browsing_context_id) {
+            Some(browsing_context) => browsing_context.pipeline_id,
+            None => return warn!("Webdriver load for closed browsing context {}.", browsing_context_id),
+        };
+        if let Some(new_pipeline_id) = self.load_url(pipeline_id, load_data, replace) {
+            self.webdriver.load_channel = Some((new_pipeline_id, reply));
         }
     }
 
@@ -2369,10 +2373,14 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
     }
 
     /// Called when the window is resized.
-    fn handle_window_size_msg(&mut self, new_size: WindowSizeData, size_type: WindowSizeType) {
+    fn handle_window_size_msg(&mut self,
+                              top_level_browsing_context_id: TopLevelBrowsingContextId,
+                              new_size: WindowSizeData,
+                              size_type: WindowSizeType)
+    {
         debug!("handle_window_size_msg: {:?}", new_size.initial_viewport.to_untyped());
 
-        let browsing_context_id = BrowsingContextId::from(self.root_browsing_context_id);
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
         self.resize_browsing_context(new_size, size_type, browsing_context_id);
 
         if let Some(resize_channel) = self.webdriver.resize_channel.take() {
@@ -2792,9 +2800,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                         let top_level_browsing_context_id = self.browsing_contexts.get(&pipeline.browsing_context_id)
                             .map(|browsing_context| browsing_context.top_level_id)
                             .unwrap_or(self.root_browsing_context_id);
+                        let url = pipeline.url.clone();
                         let can_go_forward = !self.joint_session_future_is_empty(top_level_browsing_context_id);
                         let can_go_back = !self.joint_session_past_is_empty(top_level_browsing_context_id);
-                        let url = pipeline.url.to_string();
                         let event = MozBrowserEvent::LocationChange(url, can_go_back, can_go_forward);
                         parent.trigger_mozbrowser_event(Some(top_level_browsing_context_id), event);
                     },
diff --git a/components/constellation/pipeline.rs b/components/constellation/pipeline.rs
index 85fdc54289b..1fb385d551f 100644
--- a/components/constellation/pipeline.rs
+++ b/components/constellation/pipeline.rs
@@ -52,6 +52,9 @@ pub struct Pipeline {
     /// The ID of the browsing context that contains this Pipeline.
     pub browsing_context_id: BrowsingContextId,
 
+    /// The ID of the top-level browsing context that contains this Pipeline.
+    pub top_level_browsing_context_id: TopLevelBrowsingContextId,
+
     /// The parent pipeline of this one. `None` if this is a root pipeline.
     /// Note that because of mozbrowser iframes, even top-level pipelines
     /// may have a parent (in which case the frame type will be
@@ -282,6 +285,7 @@ impl Pipeline {
 
         Ok(Pipeline::new(state.id,
                          state.browsing_context_id,
+                         state.top_level_browsing_context_id,
                          state.parent_info,
                          script_chan,
                          pipeline_chan,
@@ -295,6 +299,7 @@ impl Pipeline {
     /// spawned.
     pub fn new(id: PipelineId,
                browsing_context_id: BrowsingContextId,
+               top_level_browsing_context_id: TopLevelBrowsingContextId,
                parent_info: Option<(PipelineId, FrameType)>,
                event_loop: Rc<EventLoop>,
                layout_chan: IpcSender<LayoutControlMsg>,
@@ -306,6 +311,7 @@ impl Pipeline {
         let pipeline = Pipeline {
             id: id,
             browsing_context_id: browsing_context_id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
             parent_info: parent_info,
             event_loop: event_loop,
             layout_chan: layout_chan,
@@ -372,6 +378,7 @@ impl Pipeline {
     pub fn to_sendable(&self) -> CompositionPipeline {
         CompositionPipeline {
             id: self.id.clone(),
+            top_level_browsing_context_id: self.top_level_browsing_context_id.clone(),
             script_chan: self.event_loop.sender(),
             layout_chan: self.layout_chan.clone(),
         }
diff --git a/components/script/dom/history.rs b/components/script/dom/history.rs
index 5488d02d9bf..b9601d2ca1c 100644
--- a/components/script/dom/history.rs
+++ b/components/script/dom/history.rs
@@ -44,10 +44,9 @@ impl History {
         if !self.window.Document().is_fully_active() {
             return Err(Error::Security);
         }
-        let global_scope = self.window.upcast::<GlobalScope>();
-        let pipeline = global_scope.pipeline_id();
-        let msg = ConstellationMsg::TraverseHistory(Some(pipeline), direction);
-        let _ = global_scope.constellation_chan().send(msg);
+        let top_level_browsing_context_id = self.window.window_proxy().top_level_browsing_context_id();
+        let msg = ConstellationMsg::TraverseHistory(top_level_browsing_context_id, direction);
+        let _ = self.window.upcast::<GlobalScope>().constellation_chan().send(msg);
         Ok(())
     }
 }
@@ -58,13 +57,12 @@ impl HistoryMethods for History {
         if !self.window.Document().is_fully_active() {
             return Err(Error::Security);
         }
-        let global_scope = self.window.upcast::<GlobalScope>();
-        let pipeline = global_scope.pipeline_id();
+        let top_level_browsing_context_id = self.window.window_proxy().top_level_browsing_context_id();
         let (sender, recv) = ipc::channel().expect("Failed to create channel to send jsh length.");
-        let msg = ConstellationMsg::JointSessionHistoryLength(pipeline, sender);
-        let _ = global_scope.constellation_chan().send(msg);
+        let msg = ConstellationMsg::JointSessionHistoryLength(top_level_browsing_context_id, sender);
+        let _ = self.window.upcast::<GlobalScope>().constellation_chan().send(msg);
         Ok(recv.recv().unwrap())
-    }
+}
 
     // https://html.spec.whatwg.org/multipage/#dom-history-go
     fn Go(&self, delta: i32) -> ErrorResult {
diff --git a/components/script/dom/htmliframeelement.rs b/components/script/dom/htmliframeelement.rs
index 00755213c75..3621bf3f4f9 100644
--- a/components/script/dom/htmliframeelement.rs
+++ b/components/script/dom/htmliframeelement.rs
@@ -498,7 +498,7 @@ unsafe fn build_mozbrowser_event_detail(event: MozBrowserEvent,
         }
         MozBrowserEvent::LocationChange(url, can_go_back, can_go_forward) => {
             BrowserElementLocationChangeEventDetail {
-                url: Some(DOMString::from(url)),
+                url: Some(DOMString::from(url.as_str())),
                 canGoBack: Some(can_go_back),
                 canGoForward: Some(can_go_forward),
             }.to_jsval(cx, rval);
@@ -540,18 +540,16 @@ unsafe fn build_mozbrowser_event_detail(event: MozBrowserEvent,
 
 pub fn Navigate(iframe: &HTMLIFrameElement, direction: TraversalDirection) -> ErrorResult {
     if iframe.Mozbrowser() {
-        if iframe.upcast::<Node>().is_in_doc_with_browsing_context() {
+        if let Some(top_level_browsing_context_id) = iframe.top_level_browsing_context_id() {
             let window = window_from_node(iframe);
-            let msg = ConstellationMsg::TraverseHistory(iframe.pipeline_id(), direction);
+            let msg = ConstellationMsg::TraverseHistory(top_level_browsing_context_id, direction);
             window.upcast::<GlobalScope>().constellation_chan().send(msg).unwrap();
+            return Ok(());
         }
-
-        Ok(())
-    } else {
-        debug!(concat!("this frame is not mozbrowser: mozbrowser attribute missing, or not a top",
-            "level window, or mozbrowser preference not set (use --pref dom.mozbrowser.enabled)"));
-        Err(Error::NotSupported)
     }
+    debug!(concat!("this frame is not mozbrowser: mozbrowser attribute missing, or not a top",
+                   "level window, or mozbrowser preference not set (use --pref dom.mozbrowser.enabled)"));
+    Err(Error::NotSupported)
 }
 
 impl HTMLIFrameElementMethods for HTMLIFrameElement {
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index d66baaeda81..405616fedec 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1280,8 +1280,8 @@ impl ScriptThread {
                 webdriver_handlers::handle_get_rect(&*documents, pipeline_id, node_id, reply),
             WebDriverScriptCommand::GetElementText(node_id, reply) =>
                 webdriver_handlers::handle_get_text(&*documents, pipeline_id, node_id, reply),
-            WebDriverScriptCommand::GetPipelineId(browsing_context_id, reply) =>
-                webdriver_handlers::handle_get_pipeline_id(&*documents, pipeline_id, browsing_context_id, reply),
+            WebDriverScriptCommand::GetBrowsingContextId(webdriver_frame_id, reply) =>
+                webdriver_handlers::handle_get_browsing_context_id(&*documents, pipeline_id, webdriver_frame_id, reply),
             WebDriverScriptCommand::GetUrl(reply) =>
                 webdriver_handlers::handle_get_url(&*documents, pipeline_id, reply),
             WebDriverScriptCommand::IsEnabled(element_id, reply) =>
diff --git a/components/script/webdriver_handlers.rs b/components/script/webdriver_handlers.rs
index 7bc31263c25..1aa2ad05dab 100644
--- a/components/script/webdriver_handlers.rs
+++ b/components/script/webdriver_handlers.rs
@@ -29,6 +29,7 @@ use hyper_serde::Serde;
 use ipc_channel::ipc::{self, IpcSender};
 use js::jsapi::{HandleValue, JSContext};
 use js::jsval::UndefinedValue;
+use msg::constellation_msg::BrowsingContextId;
 use msg::constellation_msg::PipelineId;
 use net_traits::CookieSource::{HTTP, NonHTTP};
 use net_traits::CoreResourceMsg::{GetCookiesDataForUrl, SetCookieForUrl};
@@ -109,23 +110,23 @@ pub fn handle_execute_async_script(documents: &Documents,
     window.upcast::<GlobalScope>().evaluate_js_on_global_with_result(&eval, rval.handle_mut());
 }
 
-pub fn handle_get_pipeline_id(documents: &Documents,
-                              pipeline: PipelineId,
-                              webdriver_frame_id: WebDriverFrameId,
-                              reply: IpcSender<Result<Option<PipelineId>, ()>>) {
+pub fn handle_get_browsing_context_id(documents: &Documents,
+                                      pipeline: PipelineId,
+                                      webdriver_frame_id: WebDriverFrameId,
+                                      reply: IpcSender<Result<BrowsingContextId, ()>>) {
     let result = match webdriver_frame_id {
         WebDriverFrameId::Short(_) => {
             // This isn't supported yet
-            Ok(None)
+            Err(())
         },
         WebDriverFrameId::Element(x) => {
             find_node_by_unique_id(documents, pipeline, x)
-                .and_then(|node| node.downcast::<HTMLIFrameElement>().map(|elem| elem.pipeline_id()))
+                .and_then(|node| node.downcast::<HTMLIFrameElement>().and_then(|elem| elem.browsing_context_id()))
                 .ok_or(())
         },
         WebDriverFrameId::Parent => {
             documents.find_window(pipeline)
-                .map(|window| window.parent_info().map(|(parent_id, _)| parent_id))
+                .and_then(|window| window.window_proxy().parent().map(|parent| parent.browsing_context_id()))
                 .ok_or(())
         }
     };
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 98e66ab8044..c5e62b6ee81 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -602,7 +602,7 @@ pub enum MozBrowserEvent {
     /// Sent when the browser `<iframe>` starts to load a new page.
     LoadStart,
     /// Sent when a browser `<iframe>`'s location changes.
-    LocationChange(String, bool, bool),
+    LocationChange(ServoUrl, bool, bool),
     /// Sent when a new tab is opened within a browser `<iframe>` as a result of the user
     /// issuing a command to open a link target in a new tab (for example ctrl/cmd + click.)
     /// Includes the URL.
@@ -715,21 +715,20 @@ pub enum WindowSizeType {
 #[derive(Deserialize, Serialize)]
 pub enum WebDriverCommandMsg {
     /// Get the window size.
-    GetWindowSize(PipelineId, IpcSender<WindowSizeData>),
-    /// Load a URL in the pipeline with the given ID.
-    LoadUrl(PipelineId, LoadData, IpcSender<LoadStatus>),
-    /// Refresh the pipeline with the given ID.
-    Refresh(PipelineId, IpcSender<LoadStatus>),
-    /// Pass a webdriver command to the script thread of the pipeline with the
-    /// given ID for execution.
-    ScriptCommand(PipelineId, WebDriverScriptCommand),
-    /// Act as if keys were pressed in the pipeline with the given ID.
-    SendKeys(PipelineId, Vec<(Key, KeyModifiers, KeyState)>),
+    GetWindowSize(TopLevelBrowsingContextId, IpcSender<WindowSizeData>),
+    /// Load a URL in the top-level browsing context with the given ID.
+    LoadUrl(TopLevelBrowsingContextId, LoadData, IpcSender<LoadStatus>),
+    /// Refresh the top-level browsing context with the given ID.
+    Refresh(TopLevelBrowsingContextId, IpcSender<LoadStatus>),
+    /// Pass a webdriver command to the script thread of the current pipeline
+    /// of a browsing context.
+    ScriptCommand(BrowsingContextId, WebDriverScriptCommand),
+    /// Act as if keys were pressed in the browsing context with the given ID.
+    SendKeys(BrowsingContextId, Vec<(Key, KeyModifiers, KeyState)>),
     /// Set the window size.
-    SetWindowSize(PipelineId, Size2D<u32>, IpcSender<WindowSizeData>),
-    /// Take a screenshot of the window, if the pipeline with the given ID is
-    /// the root pipeline.
-    TakeScreenshot(PipelineId, IpcSender<Option<Image>>),
+    SetWindowSize(TopLevelBrowsingContextId, Size2D<u32>, IpcSender<WindowSizeData>),
+    /// Take a screenshot of the window.
+    TakeScreenshot(TopLevelBrowsingContextId, IpcSender<Option<Image>>),
 }
 
 /// Messages to the constellation.
@@ -740,10 +739,12 @@ pub enum ConstellationMsg {
     /// Request that the constellation send the BrowsingContextId corresponding to the document
     /// with the provided pipeline id
     GetBrowsingContext(PipelineId, IpcSender<Option<BrowsingContextId>>),
-    /// Request that the constellation send the current pipeline id for the provided frame
-    /// id, or for the root frame if this is None, over a provided channel.
-    /// Also returns a boolean saying whether the document has finished loading or not.
-    GetPipeline(Option<BrowsingContextId>, IpcSender<Option<PipelineId>>),
+    /// Request that the constellation send the current pipeline id for the provided
+    /// browsing context id, over a provided channel.
+    GetPipeline(BrowsingContextId, IpcSender<Option<PipelineId>>),
+    /// Request that the constellation send the current focused top-level browsing context id,
+    /// over a provided channel.
+    GetFocusTopLevelBrowsingContext(IpcSender<Option<TopLevelBrowsingContextId>>),
     /// Requests that the constellation inform the compositor of the title of the pipeline
     /// immediately.
     GetPipelineTitle(PipelineId),
@@ -755,16 +756,16 @@ pub enum ConstellationMsg {
     KeyEvent(Option<char>, Key, KeyState, KeyModifiers),
     /// Request to load a page.
     LoadUrl(PipelineId, LoadData),
-    /// Request to traverse the joint session history.
-    TraverseHistory(Option<PipelineId>, TraversalDirection),
+    /// Request to traverse the joint session history of the provided browsing context.
+    TraverseHistory(TopLevelBrowsingContextId, TraversalDirection),
     /// Inform the constellation of a window being resized.
-    WindowSize(WindowSizeData, WindowSizeType),
+    WindowSize(TopLevelBrowsingContextId, WindowSizeData, WindowSizeType),
     /// Requests that the constellation instruct layout to begin a new tick of the animation.
     TickAnimation(PipelineId, AnimationTickType),
     /// Dispatch a webdriver command
     WebDriverCommand(WebDriverCommandMsg),
-    /// Reload the current page.
-    Reload,
+    /// Reload a top-level browsing context.
+    Reload(TopLevelBrowsingContextId),
     /// A log entry, with the top-level browsing context id and thread name
     LogEntry(Option<TopLevelBrowsingContextId>, Option<String>, LogEntry),
     /// Set the WebVR thread channel.
diff --git a/components/script_traits/script_msg.rs b/components/script_traits/script_msg.rs
index 1ebe6c17b5f..9bb7053daa0 100644
--- a/components/script_traits/script_msg.rs
+++ b/components/script_traits/script_msg.rs
@@ -86,7 +86,7 @@ pub enum ScriptMsg {
     ForwardEvent(PipelineId, CompositorEvent),
     /// Requests that the constellation retrieve the current contents of the clipboard
     GetClipboardContents(IpcSender<String>),
-    /// Get the frame id for a given pipeline.
+    /// Get the browsing context id for a given pipeline.
     GetBrowsingContextId(PipelineId, IpcSender<Option<BrowsingContextId>>),
     /// Get the parent info for a given pipeline.
     GetParentInfo(PipelineId, IpcSender<Option<(PipelineId, FrameType)>>),
@@ -104,9 +104,9 @@ pub enum ScriptMsg {
     /// The first PipelineId is for the parent, the second is for the originating pipeline.
     MozBrowserEvent(PipelineId, PipelineId, MozBrowserEvent),
     /// HTMLIFrameElement Forward or Back traversal.
-    TraverseHistory(Option<PipelineId>, TraversalDirection),
+    TraverseHistory(TopLevelBrowsingContextId, TraversalDirection),
     /// Gets the length of the joint session history from the constellation.
-    JointSessionHistoryLength(PipelineId, IpcSender<u32>),
+    JointSessionHistoryLength(TopLevelBrowsingContextId, IpcSender<u32>),
     /// Favicon detected
     NewFavicon(ServoUrl),
     /// Status message to be displayed in the chrome, eg. a link URL on mouseover.
diff --git a/components/script_traits/webdriver_msg.rs b/components/script_traits/webdriver_msg.rs
index b92e32ff65c..ed7f2ea8f18 100644
--- a/components/script_traits/webdriver_msg.rs
+++ b/components/script_traits/webdriver_msg.rs
@@ -8,7 +8,7 @@ use cookie_rs::Cookie;
 use euclid::rect::Rect;
 use hyper_serde::Serde;
 use ipc_channel::ipc::IpcSender;
-use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::BrowsingContextId;
 use rustc_serialize::json::{Json, ToJson};
 use servo_url::ServoUrl;
 
@@ -31,7 +31,7 @@ pub enum WebDriverScriptCommand {
     GetElementRect(String, IpcSender<Result<Rect<f64>, ()>>),
     GetElementTagName(String, IpcSender<Result<String, ()>>),
     GetElementText(String, IpcSender<Result<String, ()>>),
-    GetPipelineId(WebDriverFrameId, IpcSender<Result<Option<PipelineId>, ()>>),
+    GetBrowsingContextId(WebDriverFrameId, IpcSender<Result<BrowsingContextId, ()>>),
     GetUrl(IpcSender<ServoUrl>),
     IsEnabled(String, IpcSender<Result<bool, ()>>),
     IsSelected(String, IpcSender<Result<bool, ()>>),
diff --git a/components/webdriver_server/lib.rs b/components/webdriver_server/lib.rs
index 404f965bbb9..8f085e507ab 100644
--- a/components/webdriver_server/lib.rs
+++ b/components/webdriver_server/lib.rs
@@ -32,7 +32,7 @@ use hyper::method::Method::{self, Post};
 use image::{DynamicImage, ImageFormat, RgbImage};
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use keys::keycodes_to_keys;
-use msg::constellation_msg::{BrowsingContextId, PipelineId, TraversalDirection};
+use msg::constellation_msg::{BrowsingContextId, TopLevelBrowsingContextId, TraversalDirection};
 use net_traits::image::base::PixelFormat;
 use regex::Captures;
 use rustc_serialize::json::{Json, ToJson};
@@ -102,7 +102,8 @@ pub fn start_server(port: u16, constellation_chan: Sender<ConstellationMsg>) {
 /// Represents the current WebDriver session and holds relevant session state.
 struct WebDriverSession {
     id: Uuid,
-    browsing_context_id: Option<BrowsingContextId>,
+    browsing_context_id: BrowsingContextId,
+    top_level_browsing_context_id: TopLevelBrowsingContextId,
 
     /// Time to wait for injected scripts to run before interrupting them.  A [`None`] value
     /// specifies that the script should run indefinitely.
@@ -117,10 +118,14 @@ struct WebDriverSession {
 }
 
 impl WebDriverSession {
-    pub fn new() -> WebDriverSession {
+    pub fn new(browsing_context_id: BrowsingContextId,
+               top_level_browsing_context_id: TopLevelBrowsingContextId)
+               -> WebDriverSession
+    {
         WebDriverSession {
             id: Uuid::new_v4(),
-            browsing_context_id: None,
+            browsing_context_id: browsing_context_id,
+            top_level_browsing_context_id: top_level_browsing_context_id,
 
             script_timeout: Some(30_000),
             load_timeout: Some(300_000),
@@ -264,33 +269,28 @@ impl Handler {
         }
     }
 
-    fn pipeline_id(&self, frame_id: Option<BrowsingContextId>) -> WebDriverResult<PipelineId> {
+    fn focus_top_level_browsing_context_id(&self) -> WebDriverResult<TopLevelBrowsingContextId> {
+        debug!("Getting focused context.");
         let interval = 20;
         let iterations = 30_000 / interval;
         let (sender, receiver) = ipc::channel().unwrap();
 
         for _ in 0..iterations {
-            let msg = ConstellationMsg::GetPipeline(frame_id, sender.clone());
+            let msg = ConstellationMsg::GetFocusTopLevelBrowsingContext(sender.clone());
             self.constellation_chan.send(msg).unwrap();
-            // Wait until the document is ready before returning the pipeline id.
+            // Wait until the document is ready before returning the top-level browsing context id.
             if let Some(x) = receiver.recv().unwrap() {
+                debug!("Focused context is {}", x);
                 return Ok(x);
             }
             thread::sleep(Duration::from_millis(interval));
         }
 
+        debug!("Timed out getting focused context.");
         Err(WebDriverError::new(ErrorStatus::Timeout,
                                 "Failed to get window handle"))
     }
 
-    fn root_pipeline(&self) -> WebDriverResult<PipelineId> {
-        self.pipeline_id(None)
-    }
-
-    fn frame_pipeline(&self) -> WebDriverResult<PipelineId> {
-        self.pipeline_id(self.session.as_ref().and_then(|session| session.browsing_context_id))
-    }
-
     fn session(&self) -> WebDriverResult<&WebDriverSession> {
         match self.session {
             Some(ref x) => Ok(x),
@@ -299,31 +299,30 @@ impl Handler {
         }
     }
 
-    fn set_browsing_context_id(&mut self, browsing_context_id: Option<BrowsingContextId>) -> WebDriverResult<()> {
+    fn session_mut(&mut self) -> WebDriverResult<&mut WebDriverSession> {
         match self.session {
-            Some(ref mut x) => {
-                x.browsing_context_id = browsing_context_id;
-                Ok(())
-            },
+            Some(ref mut x) => Ok(x),
             None => Err(WebDriverError::new(ErrorStatus::SessionNotCreated,
                                             "Session not created"))
         }
     }
 
     fn handle_new_session(&mut self) -> WebDriverResult<WebDriverResponse> {
+        debug!("new session");
         if self.session.is_none() {
-            let session = WebDriverSession::new();
+            let top_level_browsing_context_id = self.focus_top_level_browsing_context_id()?;
+            let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
+            let session = WebDriverSession::new(browsing_context_id, top_level_browsing_context_id);
             let mut capabilities = BTreeMap::new();
             capabilities.insert("browserName".to_owned(), "servo".to_json());
             capabilities.insert("browserVersion".to_owned(), "0.0.1".to_json());
             capabilities.insert("acceptInsecureCerts".to_owned(), false.to_json());
-            let rv = Ok(WebDriverResponse::NewSession(
-                NewSessionResponse::new(
-                    session.id.to_string(),
-                    Json::Object(capabilities))));
+            let response = NewSessionResponse::new(session.id.to_string(), Json::Object(capabilities));
+            debug!("new session created {}.", session.id);
             self.session = Some(session);
-            rv
+            Ok(WebDriverResponse::NewSession(response))
         } else {
+            debug!("new session failed.");
             Err(WebDriverError::new(ErrorStatus::UnknownError,
                                     "Session already created"))
         }
@@ -334,16 +333,18 @@ impl Handler {
         Ok(WebDriverResponse::Void)
     }
 
-    #[inline]
-    fn frame_script_command(&self, cmd_msg: WebDriverScriptCommand) -> WebDriverResult<()> {
-        Ok(self.constellation_chan.send(ConstellationMsg::WebDriverCommand(
-                WebDriverCommandMsg::ScriptCommand(try!(self.frame_pipeline()), cmd_msg))).unwrap())
+    fn browsing_context_script_command(&self, cmd_msg: WebDriverScriptCommand) -> WebDriverResult<()> {
+        let browsing_context_id = self.session()?.browsing_context_id;
+        let msg = ConstellationMsg::WebDriverCommand(WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd_msg));
+        self.constellation_chan.send(msg).unwrap();
+        Ok(())
     }
 
-    #[inline]
-    fn root_script_command(&self, cmd_msg: WebDriverScriptCommand) -> WebDriverResult<()> {
-        Ok(self.constellation_chan.send(ConstellationMsg::WebDriverCommand(
-                WebDriverCommandMsg::ScriptCommand(try!(self.root_pipeline()), cmd_msg))).unwrap())
+    fn top_level_script_command(&self, cmd_msg: WebDriverScriptCommand) -> WebDriverResult<()> {
+        let browsing_context_id = BrowsingContextId::from(self.session()?.top_level_browsing_context_id);
+        let msg = ConstellationMsg::WebDriverCommand(WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd_msg));
+        self.constellation_chan.send(msg).unwrap();
+        Ok(())
     }
 
     fn handle_get(&self, parameters: &GetParameters) -> WebDriverResult<WebDriverResponse> {
@@ -353,12 +354,12 @@ impl Handler {
                                                "Invalid URL"))
         };
 
-        let pipeline_id = try!(self.root_pipeline());
+        let top_level_browsing_context_id = self.session()?.top_level_browsing_context_id;
 
         let (sender, receiver) = ipc::channel().unwrap();
 
-        let load_data = LoadData::new(url, Some(pipeline_id), None, None);
-        let cmd_msg = WebDriverCommandMsg::LoadUrl(pipeline_id, load_data, sender.clone());
+        let load_data = LoadData::new(url, None, None, None);
+        let cmd_msg = WebDriverCommandMsg::LoadUrl(top_level_browsing_context_id, load_data, sender.clone());
         self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
         self.wait_for_load(sender, receiver)
@@ -368,15 +369,10 @@ impl Handler {
                      sender: IpcSender<LoadStatus>,
                      receiver: IpcReceiver<LoadStatus>)
                      -> WebDriverResult<WebDriverResponse> {
-        let session = try!(self.session
-            .as_ref()
-            .ok_or(WebDriverError::new(ErrorStatus::SessionNotCreated, "")));
-
-        let timeout = session.load_timeout;
-        let timeout_chan = sender;
+        let timeout = self.session()?.load_timeout;
         thread::spawn(move || {
             thread::sleep(Duration::from_millis(timeout.unwrap()));
-            let _ = timeout_chan.send(LoadStatus::LoadTimeout);
+            let _ = sender.send(LoadStatus::LoadTimeout);
         });
 
         // wait to get a load event
@@ -390,7 +386,7 @@ impl Handler {
 
     fn handle_current_url(&self) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.root_script_command(WebDriverScriptCommand::GetUrl(sender)));
+        self.top_level_script_command(WebDriverScriptCommand::GetUrl(sender))?;
 
         let url = receiver.recv().unwrap();
 
@@ -399,8 +395,8 @@ impl Handler {
 
     fn handle_window_size(&self) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        let pipeline_id = try!(self.root_pipeline());
-        let cmd_msg = WebDriverCommandMsg::GetWindowSize(pipeline_id, sender);
+        let top_level_browsing_context_id = self.session()?.top_level_browsing_context_id;
+        let cmd_msg = WebDriverCommandMsg::GetWindowSize(top_level_browsing_context_id, sender);
 
         self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
@@ -413,8 +409,8 @@ impl Handler {
     fn handle_set_window_size(&self, params: &WindowSizeParameters) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
         let size = Size2D::new(params.width as u32, params.height as u32);
-        let pipeline_id = try!(self.root_pipeline());
-        let cmd_msg = WebDriverCommandMsg::SetWindowSize(pipeline_id, size, sender.clone());
+        let top_level_browsing_context_id = self.session()?.top_level_browsing_context_id;
+        let cmd_msg = WebDriverCommandMsg::SetWindowSize(top_level_browsing_context_id, size, sender.clone());
 
         self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
@@ -424,7 +420,7 @@ impl Handler {
             // On timeout, we send a GetWindowSize message to the constellation,
             // which will give the current window size.
             thread::sleep(Duration::from_millis(timeout as u64));
-            let cmd_msg = WebDriverCommandMsg::GetWindowSize(pipeline_id, sender);
+            let cmd_msg = WebDriverCommandMsg::GetWindowSize(top_level_browsing_context_id, sender);
             constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
         });
 
@@ -437,7 +433,7 @@ impl Handler {
     fn handle_is_enabled(&self, element: &WebElement) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
 
-        try!(self.root_script_command(WebDriverScriptCommand::IsEnabled(element.id.clone(), sender)));
+        self.top_level_script_command(WebDriverScriptCommand::IsEnabled(element.id.clone(), sender))?;
 
         match receiver.recv().unwrap() {
             Ok(is_enabled) => Ok(WebDriverResponse::Generic(ValueResponse::new(is_enabled.to_json()))),
@@ -448,7 +444,7 @@ impl Handler {
     fn handle_is_selected(&self, element: &WebElement) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
 
-        try!(self.root_script_command(WebDriverScriptCommand::IsSelected(element.id.clone(), sender)));
+        self.top_level_script_command(WebDriverScriptCommand::IsSelected(element.id.clone(), sender))?;
 
         match receiver.recv().unwrap() {
             Ok(is_selected) => Ok(WebDriverResponse::Generic(ValueResponse::new(is_selected.to_json()))),
@@ -457,21 +453,27 @@ impl Handler {
     }
 
     fn handle_go_back(&self) -> WebDriverResult<WebDriverResponse> {
-        self.constellation_chan.send(ConstellationMsg::TraverseHistory(None, TraversalDirection::Back(1))).unwrap();
+        let top_level_browsing_context_id = self.session()?.top_level_browsing_context_id;
+        let direction = TraversalDirection::Back(1);
+        let msg = ConstellationMsg::TraverseHistory(top_level_browsing_context_id, direction);
+        self.constellation_chan.send(msg).unwrap();
         Ok(WebDriverResponse::Void)
     }
 
     fn handle_go_forward(&self) -> WebDriverResult<WebDriverResponse> {
-        self.constellation_chan.send(ConstellationMsg::TraverseHistory(None, TraversalDirection::Forward(1))).unwrap();
+        let top_level_browsing_context_id = self.session()?.top_level_browsing_context_id;
+        let direction = TraversalDirection::Forward(1);
+        let msg = ConstellationMsg::TraverseHistory(top_level_browsing_context_id, direction);
+        self.constellation_chan.send(msg).unwrap();
         Ok(WebDriverResponse::Void)
     }
 
     fn handle_refresh(&self) -> WebDriverResult<WebDriverResponse> {
-        let pipeline_id = try!(self.root_pipeline());
+        let top_level_browsing_context_id = self.session()?.top_level_browsing_context_id;
 
         let (sender, receiver) = ipc::channel().unwrap();
 
-        let cmd_msg = WebDriverCommandMsg::Refresh(pipeline_id, sender.clone());
+        let cmd_msg = WebDriverCommandMsg::Refresh(top_level_browsing_context_id, sender.clone());
         self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
         self.wait_for_load(sender, receiver)
@@ -480,7 +482,7 @@ impl Handler {
     fn handle_title(&self) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
 
-        try!(self.root_script_command(WebDriverScriptCommand::GetTitle(sender)));
+        self.top_level_script_command(WebDriverScriptCommand::GetTitle(sender))?;
 
         let value = receiver.recv().unwrap();
         Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json())))
@@ -508,8 +510,8 @@ impl Handler {
 
         let (sender, receiver) = ipc::channel().unwrap();
 
-        try!(self.frame_script_command(WebDriverScriptCommand::FindElementCSS(parameters.value.clone(),
-                                                                              sender)));
+        let cmd = WebDriverScriptCommand::FindElementCSS(parameters.value.clone(), sender);
+        self.browsing_context_script_command(cmd)?;
 
         match receiver.recv().unwrap() {
             Ok(value) => {
@@ -525,8 +527,9 @@ impl Handler {
         use webdriver::common::FrameId;
         let frame_id = match parameters.id {
             FrameId::Null => {
-                self.set_browsing_context_id(None).unwrap();
-                return Ok(WebDriverResponse::Void)
+                let session = self.session_mut()?;
+                session.browsing_context_id = BrowsingContextId::from(session.top_level_browsing_context_id);
+                return Ok(WebDriverResponse::Void);
             },
             FrameId::Short(ref x) => WebDriverFrameId::Short(*x),
             FrameId::Element(ref x) => WebDriverFrameId::Element(x.id.clone())
@@ -545,28 +548,15 @@ impl Handler {
             return Err(WebDriverError::new(ErrorStatus::UnsupportedOperation,
                                            "Selecting frame by id not supported"));
         }
-        let pipeline_id = try!(self.frame_pipeline());
+
         let (sender, receiver) = ipc::channel().unwrap();
-        let cmd = WebDriverScriptCommand::GetPipelineId(frame_id, sender);
-        {
-            self.constellation_chan.send(ConstellationMsg::WebDriverCommand(
-                WebDriverCommandMsg::ScriptCommand(pipeline_id, cmd))).unwrap();
-        }
+        let cmd = WebDriverScriptCommand::GetBrowsingContextId(frame_id, sender);
+        self.browsing_context_script_command(cmd)?;
 
-        let context_id = match receiver.recv().unwrap() {
-            Ok(Some(pipeline_id)) => {
-                let (sender, receiver) = ipc::channel().unwrap();
-                self.constellation_chan.send(ConstellationMsg::GetBrowsingContext(pipeline_id, sender)).unwrap();
-                receiver.recv().unwrap()
-            },
-            Ok(None) => None,
-            Err(_) => {
-                return Err(WebDriverError::new(ErrorStatus::NoSuchFrame,
-                                               "Frame does not exist"));
-            }
-        };
+        let browsing_context_id = receiver.recv().unwrap()
+            .or(Err(WebDriverError::new(ErrorStatus::NoSuchFrame, "Frame does not exist")))?;
 
-        self.set_browsing_context_id(context_id).unwrap();
+        self.session_mut()?.browsing_context_id = browsing_context_id;
         Ok(WebDriverResponse::Void)
     }
 
@@ -578,8 +568,8 @@ impl Handler {
         }
 
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::FindElementsCSS(parameters.value.clone(),
-                                                                               sender)));
+        let cmd = WebDriverScriptCommand::FindElementsCSS(parameters.value.clone(), sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(value) => {
                 let resp_value: Vec<Json> = value.into_iter().map(
@@ -594,7 +584,8 @@ impl Handler {
     // https://w3c.github.io/webdriver/webdriver-spec.html#get-element-rect
     fn handle_element_rect(&self, element: &WebElement) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetElementRect(element.id.clone(), sender)));
+        let cmd = WebDriverScriptCommand::GetElementRect(element.id.clone(), sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(rect) => {
                 let response = ElementRectResponse::new(rect.origin.x, rect.origin.y,
@@ -608,7 +599,8 @@ impl Handler {
 
     fn handle_element_text(&self, element: &WebElement) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetElementText(element.id.clone(), sender)));
+        let cmd = WebDriverScriptCommand::GetElementText(element.id.clone(), sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(value) => Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json()))),
             Err(_) => Err(WebDriverError::new(ErrorStatus::StaleElementReference,
@@ -618,14 +610,16 @@ impl Handler {
 
     fn handle_active_element(&self) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetActiveElement(sender)));
+        let cmd = WebDriverScriptCommand::GetActiveElement(sender);
+        self.browsing_context_script_command(cmd)?;
         let value = receiver.recv().unwrap().map(|x| WebElement::new(x).to_json());
         Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json())))
     }
 
     fn handle_element_tag_name(&self, element: &WebElement) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetElementTagName(element.id.clone(), sender)));
+        let cmd = WebDriverScriptCommand::GetElementTagName(element.id.clone(), sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(value) => Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json()))),
             Err(_) => Err(WebDriverError::new(ErrorStatus::StaleElementReference,
@@ -635,8 +629,8 @@ impl Handler {
 
     fn handle_element_attribute(&self, element: &WebElement, name: &str) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetElementAttribute(element.id.clone(), name.to_owned(),
-                                                                                   sender)));
+        let cmd = WebDriverScriptCommand::GetElementAttribute(element.id.clone(), name.to_owned(), sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(value) => Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json()))),
             Err(_) => Err(WebDriverError::new(ErrorStatus::StaleElementReference,
@@ -646,8 +640,8 @@ impl Handler {
 
     fn handle_element_css(&self, element: &WebElement, name: &str) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetElementCSS(element.id.clone(), name.to_owned(),
-                                                                             sender)));
+        let cmd = WebDriverScriptCommand::GetElementCSS(element.id.clone(), name.to_owned(), sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(value) => Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json()))),
             Err(_) => Err(WebDriverError::new(ErrorStatus::StaleElementReference,
@@ -657,7 +651,8 @@ impl Handler {
 
     fn handle_get_cookies(&self) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetCookies(sender)));
+        let cmd = WebDriverScriptCommand::GetCookies(sender);
+        self.browsing_context_script_command(cmd)?;
         let cookies = receiver.recv().unwrap();
         let response = cookies.into_iter().map(|cookie| {
             cookie_msg_to_cookie(cookie.into_inner())
@@ -667,7 +662,8 @@ impl Handler {
 
     fn handle_get_cookie(&self, name: &str) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
-        try!(self.frame_script_command(WebDriverScriptCommand::GetCookie(name.to_owned(), sender)));
+        let cmd = WebDriverScriptCommand::GetCookie(name.to_owned(), sender);
+        self.browsing_context_script_command(cmd)?;
         let cookies = receiver.recv().unwrap();
         let response = cookies.into_iter().map(|cookie| {
             cookie_msg_to_cookie(cookie.into_inner())
@@ -690,7 +686,8 @@ impl Handler {
             _ => cookie.finish(),
         };
 
-        try!(self.frame_script_command(WebDriverScriptCommand::AddCookie(cookie, sender)));
+        let cmd = WebDriverScriptCommand::AddCookie(cookie, sender);
+        self.browsing_context_script_command(cmd)?;
         match receiver.recv().unwrap() {
             Ok(_) => Ok(WebDriverResponse::Void),
             Err(response) => match response {
@@ -728,20 +725,18 @@ impl Handler {
 
         let (sender, receiver) = ipc::channel().unwrap();
         let command = WebDriverScriptCommand::ExecuteScript(script, sender);
-        self.execute_script(command, receiver)
+        self.browsing_context_script_command(command)?;
+        let result = receiver.recv().unwrap();
+        self.postprocess_js_result(result)
     }
 
     fn handle_execute_async_script(&self,
                                    parameters: &JavascriptCommandParameters)
                                    -> WebDriverResult<WebDriverResponse> {
-        let session = try!(self.session
-            .as_ref()
-            .ok_or(WebDriverError::new(ErrorStatus::SessionNotCreated, "")));
-
         let func_body = &parameters.script;
         let args_string = "window.webdriverCallback";
 
-        let script = match session.script_timeout {
+        let script = match self.session()?.script_timeout {
             Some(timeout) => {
                 format!("setTimeout(webdriverTimeout, {}); (function(callback) {{ {} }})({})",
                         timeout,
@@ -753,19 +748,13 @@ impl Handler {
 
         let (sender, receiver) = ipc::channel().unwrap();
         let command = WebDriverScriptCommand::ExecuteAsyncScript(script, sender);
-        self.execute_script(command, receiver)
+        self.browsing_context_script_command(command)?;
+        let result = receiver.recv().unwrap();
+        self.postprocess_js_result(result)
     }
 
-    fn execute_script(&self,
-                      command: WebDriverScriptCommand,
-                      receiver: IpcReceiver<WebDriverJSResult>)
-                      -> WebDriverResult<WebDriverResponse> {
-        let pipeline_id = try!(self.frame_pipeline());
-
-        let cmd_msg = WebDriverCommandMsg::ScriptCommand(pipeline_id, command);
-        self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
-
-        match receiver.recv().unwrap() {
+    fn postprocess_js_result(&self, result: WebDriverJSResult) -> WebDriverResult<WebDriverResponse> {
+        match result {
             Ok(value) => Ok(WebDriverResponse::Generic(ValueResponse::new(value.to_json()))),
             Err(WebDriverJSError::Timeout) => Err(WebDriverError::new(ErrorStatus::Timeout, "")),
             Err(WebDriverJSError::UnknownType) => Err(WebDriverError::new(
@@ -778,12 +767,12 @@ impl Handler {
     fn handle_element_send_keys(&self,
                                 element: &WebElement,
                                 keys: &SendKeysParameters) -> WebDriverResult<WebDriverResponse> {
-        let pipeline_id = try!(self.frame_pipeline());
+        let browsing_context_id = self.session()?.browsing_context_id;
 
         let (sender, receiver) = ipc::channel().unwrap();
 
         let cmd = WebDriverScriptCommand::FocusElement(element.id.clone(), sender);
-        let cmd_msg = WebDriverCommandMsg::ScriptCommand(pipeline_id, cmd);
+        let cmd_msg = WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd);
         self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
         // TODO: distinguish the not found and not focusable cases
@@ -793,7 +782,9 @@ impl Handler {
         let keys = try!(keycodes_to_keys(&keys.value).or_else(|_|
             Err(WebDriverError::new(ErrorStatus::UnsupportedOperation, "Failed to convert keycodes"))));
 
-        let cmd_msg = WebDriverCommandMsg::SendKeys(pipeline_id, keys);
+        // TODO: there's a race condition caused by these being two separate messages,
+        // so the constellation may have changed state between them.
+        let cmd_msg = WebDriverCommandMsg::SendKeys(browsing_context_id, keys);
         self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
         Ok(WebDriverResponse::Void)
@@ -801,14 +792,14 @@ impl Handler {
 
     fn handle_take_screenshot(&self) -> WebDriverResult<WebDriverResponse> {
         let mut img = None;
-        let pipeline_id = try!(self.root_pipeline());
+        let top_level_id = self.session()?.top_level_browsing_context_id;
 
         let interval = 1000;
         let iterations = 30_000 / interval;
 
         for _ in 0..iterations {
             let (sender, receiver) = ipc::channel().unwrap();
-            let cmd_msg = WebDriverCommandMsg::TakeScreenshot(pipeline_id, sender);
+            let cmd_msg = WebDriverCommandMsg::TakeScreenshot(top_level_id, sender);
             self.constellation_chan.send(ConstellationMsg::WebDriverCommand(cmd_msg)).unwrap();
 
             if let Some(x) = receiver.recv().unwrap() {