compositor.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

use CompositionPipeline;
use SendableFrameTree;
use compositor_thread::{CompositorProxy, CompositorReceiver};
use compositor_thread::{InitialCompositorState, Msg, RenderListener};
use delayed_composition::DelayedCompositionTimerProxy;
use euclid::Point2D;
use euclid::point::TypedPoint2D;
use euclid::scale_factor::ScaleFactor;
use euclid::size::TypedSize2D;
use gfx_traits::{DevicePixel, ScrollRootId};
use gfx_traits::{Epoch, FragmentType};
use gleam::gl;
use gleam::gl::types::{GLint, GLsizei};
use image::{DynamicImage, ImageFormat, RgbImage};
use ipc_channel::ipc::{self, IpcSender, IpcSharedMemory};
use msg::constellation_msg::{Key, KeyModifiers, KeyState, CONTROL};
use msg::constellation_msg::{PipelineId, PipelineIndex, PipelineNamespaceId, TraversalDirection};
use net_traits::image::base::{Image, PixelFormat};
use profile_traits::time::{self, ProfilerCategory, profile};
use script_traits::{AnimationState, AnimationTickType, ConstellationControlMsg};
use script_traits::{ConstellationMsg, LayoutControlMsg, LoadData, MouseButton};
use script_traits::{MouseEventType, StackingContextScrollState};
use script_traits::{TouchpadPressurePhase, TouchEventType, TouchId, WindowSizeData, WindowSizeType};
use script_traits::CompositorEvent::{self, MouseMoveEvent, MouseButtonEvent, TouchEvent, TouchpadPressureEvent};
use servo_config::opts;
use servo_config::prefs::PREFS;
use servo_geometry::ScreenPx;
use servo_url::ServoUrl;
use std::collections::HashMap;
use std::fs::File;
use std::rc::Rc;
use std::sync::mpsc::Sender;
use std::time::{Duration, Instant};
use style_traits::{PagePx, ViewportPx};
use style_traits::viewport::ViewportConstraints;
use time::{precise_time_ns, precise_time_s};
use touch::{TouchHandler, TouchAction};
use webrender;
use webrender_traits::{self, ScrollEventPhase, ServoScrollRootId, LayoutPoint};
use windowing::{self, MouseWindowEvent, WindowEvent, WindowMethods, WindowNavigateMsg};

#[derive(Debug, PartialEq)]
enum UnableToComposite {
    WindowUnprepared,
    NotReadyToPaintImage(NotReadyToPaint),
}

#[derive(Debug, PartialEq)]
enum NotReadyToPaint {
    AnimationsActive,
    JustNotifiedConstellation,
    WaitingOnConstellation,
}

// Default viewport constraints
const MAX_ZOOM: f32 = 8.0;
const MIN_ZOOM: f32 = 0.1;

trait ConvertPipelineIdFromWebRender {
    fn from_webrender(&self) -> PipelineId;
}

impl ConvertPipelineIdFromWebRender for webrender_traits::PipelineId {
    fn from_webrender(&self) -> PipelineId {
        PipelineId {
            namespace_id: PipelineNamespaceId(self.0),
            index: PipelineIndex(self.1),
        }
    }
}

trait ConvertScrollRootIdFromWebRender {
    fn from_webrender(&self) -> ScrollRootId;
}

impl ConvertScrollRootIdFromWebRender for webrender_traits::ServoScrollRootId {
    fn from_webrender(&self) -> ScrollRootId {
        ScrollRootId(self.0)
    }
}

trait ConvertFragmentTypeFromWebRender {
    fn from_webrender(&self) -> FragmentType;
}

impl ConvertFragmentTypeFromWebRender for webrender_traits::FragmentType {
    fn from_webrender(&self) -> FragmentType {
        match *self {
            webrender_traits::FragmentType::FragmentBody => FragmentType::FragmentBody,
            webrender_traits::FragmentType::BeforePseudoContent => {
                FragmentType::BeforePseudoContent
            }
            webrender_traits::FragmentType::AfterPseudoContent => FragmentType::AfterPseudoContent,
        }
    }
}

/// Holds the state when running reftests that determines when it is
/// safe to save the output image.
#[derive(Copy, Clone, PartialEq)]
enum ReadyState {
    Unknown,
    WaitingForConstellationReply,
    ReadyToSaveImage,
}

#[derive(PartialEq, Eq, Debug, Copy, Clone)]
struct FrameTreeId(u32);

impl FrameTreeId {
    pub fn next(&mut self) {
        self.0 += 1;
    }
}

/// One pixel in layer coordinate space.
///
/// This unit corresponds to a "pixel" in layer coordinate space, which after scaling and
/// transformation becomes a device pixel.
#[derive(Copy, Clone, Debug)]
enum LayerPixel {}

/// NB: Never block on the constellation, because sometimes the constellation blocks on us.
pub struct IOCompositor<Window: WindowMethods> {
    /// The application window.
    window: Rc<Window>,

    /// The port on which we receive messages.
    port: Box<CompositorReceiver>,

    /// The root pipeline.
    root_pipeline: Option<CompositionPipeline>,

    /// Tracks details about each active pipeline that the compositor knows about.
    pipeline_details: HashMap<PipelineId, PipelineDetails>,

    /// The scene scale, to allow for zooming and high-resolution painting.
    scale: ScaleFactor<f32, LayerPixel, DevicePixel>,

    /// The application window size.
    window_size: TypedSize2D<u32, DevicePixel>,

    /// The overridden viewport.
    viewport: Option<(TypedPoint2D<u32, DevicePixel>, TypedSize2D<u32, DevicePixel>)>,

    /// "Mobile-style" zoom that does not reflow the page.
    viewport_zoom: ScaleFactor<f32, PagePx, ViewportPx>,

    /// Viewport zoom constraints provided by @viewport.
    min_viewport_zoom: Option<ScaleFactor<f32, PagePx, ViewportPx>>,
    max_viewport_zoom: Option<ScaleFactor<f32, PagePx, ViewportPx>>,

    /// "Desktop-style" zoom that resizes the viewport to fit the window.
    /// See `ViewportPx` docs in util/geom.rs for details.
    page_zoom: ScaleFactor<f32, ViewportPx, ScreenPx>,

    /// The device pixel ratio for this window.
    scale_factor: ScaleFactor<f32, ScreenPx, DevicePixel>,

    channel_to_self: Box<CompositorProxy + Send>,

    /// A handle to the delayed composition timer.
    delayed_composition_timer: DelayedCompositionTimerProxy,

    /// The type of composition to perform
    composite_target: CompositeTarget,

    /// Tracks whether we should composite this frame.
    composition_request: CompositionRequest,

    /// Tracks whether we are in the process of shutting down, or have shut down and should close
    /// the compositor.
    shutdown_state: ShutdownState,

    /// Tracks the last composite time.
    last_composite_time: u64,

    /// Tracks whether the zoom action has happened recently.
    zoom_action: bool,

    /// The time of the last zoom action has started.
    zoom_time: f64,

    /// Whether the page being rendered has loaded completely.
    /// Differs from ReadyState because we can finish loading (ready)
    /// many times for a single page.
    got_load_complete_message: bool,

    /// The current frame tree ID (used to reject old paint buffers)
    frame_tree_id: FrameTreeId,

    /// The channel on which messages can be sent to the constellation.
    constellation_chan: Sender<ConstellationMsg>,

    /// The channel on which messages can be sent to the time profiler.
    time_profiler_chan: time::ProfilerChan,

    /// Touch input state machine
    touch_handler: TouchHandler,

    /// Pending scroll/zoom events.
    pending_scroll_zoom_events: Vec<ScrollZoomEvent>,

    /// Whether we're waiting on a recomposite after dispatching a scroll.
    waiting_for_results_of_scroll: bool,

    /// Used by the logic that determines when it is safe to output an
    /// image for the reftest framework.
    ready_to_save_state: ReadyState,

    /// Whether a scroll is in progress; i.e. whether the user's fingers are down.
    scroll_in_progress: bool,

    in_scroll_transaction: Option<Instant>,

    /// The webrender renderer.
    webrender: webrender::Renderer,

    /// The webrender interface, if enabled.
    webrender_api: webrender_traits::RenderApi,
}

#[derive(Copy, Clone)]
struct ScrollZoomEvent {
    /// Change the pinch zoom level by this factor
    magnification: f32,
    /// Scroll by this offset
    delta: TypedPoint2D<f32, DevicePixel>,
    /// Apply changes to the frame at this location
    cursor: TypedPoint2D<i32, DevicePixel>,
    /// The scroll event phase.
    phase: ScrollEventPhase,
    /// The number of OS events that have been coalesced together into this one event.
    event_count: u32,
}

#[derive(PartialEq, Debug)]
enum CompositionRequest {
    NoCompositingNecessary,
    DelayedComposite(u64),
    CompositeNow(CompositingReason),
}

#[derive(Clone, Copy, PartialEq, Debug)]
enum ShutdownState {
    NotShuttingDown,
    ShuttingDown,
    FinishedShuttingDown,
}

struct PipelineDetails {
    /// The pipeline associated with this PipelineDetails object.
    pipeline: Option<CompositionPipeline>,

    /// Whether animations are running
    animations_running: bool,

    /// Whether there are animation callbacks
    animation_callbacks_running: bool,

    /// Whether this pipeline is visible
    visible: bool,
}

impl PipelineDetails {
    fn new() -> PipelineDetails {
        PipelineDetails {
            pipeline: None,
            animations_running: false,
            animation_callbacks_running: false,
            visible: true,
        }
    }
}

#[derive(Clone, Copy, PartialEq, Debug)]
enum CompositeTarget {
    /// Normal composition to a window
    Window,

    /// Compose as normal, but also return a PNG of the composed output
    WindowAndPng,

    /// Compose to a PNG, write it to disk, and then exit the browser (used for reftests)
    PngFile
}

struct RenderTargetInfo {
    framebuffer_ids: Vec<gl::GLuint>,
    renderbuffer_ids: Vec<gl::GLuint>,
    texture_ids: Vec<gl::GLuint>,
}

impl RenderTargetInfo {
    fn empty() -> RenderTargetInfo {
        RenderTargetInfo {
            framebuffer_ids: Vec::new(),
            renderbuffer_ids: Vec::new(),
            texture_ids: Vec::new(),
        }
    }
}

fn initialize_png(width: usize, height: usize) -> RenderTargetInfo {
    let framebuffer_ids = gl::gen_framebuffers(1);
    gl::bind_framebuffer(gl::FRAMEBUFFER, framebuffer_ids[0]);

    let texture_ids = gl::gen_textures(1);
    gl::bind_texture(gl::TEXTURE_2D, texture_ids[0]);

    gl::tex_image_2d(gl::TEXTURE_2D, 0, gl::RGB as GLint, width as GLsizei,
                     height as GLsizei, 0, gl::RGB, gl::UNSIGNED_BYTE, None);
    gl::tex_parameter_i(gl::TEXTURE_2D, gl::TEXTURE_MAG_FILTER, gl::NEAREST as GLint);
    gl::tex_parameter_i(gl::TEXTURE_2D, gl::TEXTURE_MIN_FILTER, gl::NEAREST as GLint);

    gl::framebuffer_texture_2d(gl::FRAMEBUFFER, gl::COLOR_ATTACHMENT0, gl::TEXTURE_2D,
                               texture_ids[0], 0);

    gl::bind_texture(gl::TEXTURE_2D, 0);

    let renderbuffer_ids = gl::gen_renderbuffers(1);
    let depth_rb = renderbuffer_ids[0];
    gl::bind_renderbuffer(gl::RENDERBUFFER, depth_rb);
    gl::renderbuffer_storage(gl::RENDERBUFFER,
                             gl::DEPTH_COMPONENT24,
                             width as gl::GLsizei,
                             height as gl::GLsizei);
    gl::framebuffer_renderbuffer(gl::FRAMEBUFFER,
                                 gl::DEPTH_ATTACHMENT,
                                 gl::RENDERBUFFER,
                                 depth_rb);

    RenderTargetInfo {
        framebuffer_ids: framebuffer_ids,
        renderbuffer_ids: renderbuffer_ids,
        texture_ids: texture_ids,
    }
}

struct RenderNotifier {
    compositor_proxy: Box<CompositorProxy>,
    constellation_chan: Sender<ConstellationMsg>,
}

impl RenderNotifier {
    fn new(compositor_proxy: Box<CompositorProxy>,
           constellation_chan: Sender<ConstellationMsg>) -> RenderNotifier {
        RenderNotifier {
            compositor_proxy: compositor_proxy,
            constellation_chan: constellation_chan,
        }
    }
}

impl webrender_traits::RenderNotifier for RenderNotifier {
    fn new_frame_ready(&mut self) {
        self.compositor_proxy.recomposite(CompositingReason::NewWebRenderFrame);
    }

    fn new_scroll_frame_ready(&mut self, composite_needed: bool) {
        self.compositor_proxy.send(Msg::NewScrollFrameReady(composite_needed));
    }

    fn pipeline_size_changed(&mut self,
                             pipeline_id: webrender_traits::PipelineId,
                             size: Option<webrender_traits::LayoutSize>) {
        let pipeline_id = pipeline_id.from_webrender();

        if let Some(size) = size {
            let msg = ConstellationMsg::FrameSize(pipeline_id, size.to_untyped());
            if let Err(e) = self.constellation_chan.send(msg) {
                warn!("Compositor resize to constellation failed ({}).", e);
            }
        }
    }
}

// Used to dispatch functions from webrender to the main thread's event loop.
struct CompositorThreadDispatcher {
    compositor_proxy: Box<CompositorProxy>
}

impl webrender_traits::RenderDispatcher for CompositorThreadDispatcher {
    fn dispatch(&self, f: Box<Fn() + Send>) {
        self.compositor_proxy.send(Msg::Dispatch(f));
    }
}

impl<Window: WindowMethods> IOCompositor<Window> {
    fn new(window: Rc<Window>, state: InitialCompositorState)
           -> IOCompositor<Window> {
        let window_size = window.framebuffer_size();
        let scale_factor = window.scale_factor();
        let composite_target = match opts::get().output_file {
            Some(_) => CompositeTarget::PngFile,
            None => CompositeTarget::Window
        };

        IOCompositor {
            window: window,
            port: state.receiver,
            root_pipeline: None,
            pipeline_details: HashMap::new(),
            window_size: window_size,
            scale: ScaleFactor::new(1.0),
            viewport: None,
            scale_factor: scale_factor,
            channel_to_self: state.sender.clone_compositor_proxy(),
            delayed_composition_timer: DelayedCompositionTimerProxy::new(state.sender),
            composition_request: CompositionRequest::NoCompositingNecessary,
            touch_handler: TouchHandler::new(),
            pending_scroll_zoom_events: Vec::new(),
            waiting_for_results_of_scroll: false,
            composite_target: composite_target,
            shutdown_state: ShutdownState::NotShuttingDown,
            page_zoom: ScaleFactor::new(1.0),
            viewport_zoom: ScaleFactor::new(1.0),
            min_viewport_zoom: None,
            max_viewport_zoom: None,
            zoom_action: false,
            zoom_time: 0f64,
            got_load_complete_message: false,
            frame_tree_id: FrameTreeId(0),
            constellation_chan: state.constellation_chan,
            time_profiler_chan: state.time_profiler_chan,
            last_composite_time: 0,
            ready_to_save_state: ReadyState::Unknown,
            scroll_in_progress: false,
            in_scroll_transaction: None,
            webrender: state.webrender,
            webrender_api: state.webrender_api_sender.create_api(),
        }
    }

    pub fn create(window: Rc<Window>, state: InitialCompositorState) -> IOCompositor<Window> {
        let mut compositor = IOCompositor::new(window, state);

        let compositor_proxy_for_webrender = compositor.channel_to_self
                                                       .clone_compositor_proxy();
        let render_notifier = RenderNotifier::new(compositor_proxy_for_webrender,
                                                  compositor.constellation_chan.clone());
        compositor.webrender.set_render_notifier(Box::new(render_notifier));

        if cfg!(target_os = "windows") {
            // Used to dispatch functions from webrender to the main thread's event loop.
            // Required to allow WGL GLContext sharing in Windows.
            let dispatcher = Box::new(CompositorThreadDispatcher {
                compositor_proxy: compositor.channel_to_self.clone_compositor_proxy()
            });
            compositor.webrender.set_main_thread_dispatcher(dispatcher);
        }

        // Set the size of the root layer.
        compositor.update_zoom_transform();

        // Tell the constellation about the initial window size.
        compositor.send_window_size(WindowSizeType::Initial);

        compositor
    }

    fn start_shutting_down(&mut self) {
        debug!("Compositor sending Exit message to Constellation");
        if let Err(e) = self.constellation_chan.send(ConstellationMsg::Exit) {
            warn!("Sending exit message to constellation failed ({}).", e);
        }

        self.shutdown_state = ShutdownState::ShuttingDown;
    }

    fn finish_shutting_down(&mut self) {
        debug!("Compositor received message that constellation shutdown is complete");

        // Drain compositor port, sometimes messages contain channels that are blocking
        // another thread from finishing (i.e. SetFrameTree).
        while self.port.try_recv_compositor_msg().is_some() {}

        // Tell the profiler, memory profiler, and scrolling timer to shut down.
        if let Ok((sender, receiver)) = ipc::channel() {
            self.time_profiler_chan.send(time::ProfilerMsg::Exit(sender));
            let _ = receiver.recv();
        }

        self.delayed_composition_timer.shutdown();

        self.shutdown_state = ShutdownState::FinishedShuttingDown;
    }

    fn handle_browser_message(&mut self, msg: Msg) -> bool {
        match (msg, self.shutdown_state) {
            (_, ShutdownState::FinishedShuttingDown) => {
                error!("compositor shouldn't be handling messages after shutting down");
                return false
            }

            (Msg::Exit, _) => {
                self.start_shutting_down();
            }

            (Msg::ShutdownComplete, _) => {
                self.finish_shutting_down();
                return false;
            }

            (Msg::ChangeRunningAnimationsState(pipeline_id, animation_state),
             ShutdownState::NotShuttingDown) => {
                self.change_running_animations_state(pipeline_id, animation_state);
            }

            (Msg::ChangePageTitle(pipeline_id, title), ShutdownState::NotShuttingDown) => {
                self.change_page_title(pipeline_id, title);
            }

            (Msg::ChangePageUrl(pipeline_id, url), ShutdownState::NotShuttingDown) => {
                self.change_page_url(pipeline_id, url);
            }

            (Msg::SetFrameTree(frame_tree, response_chan),
             ShutdownState::NotShuttingDown) => {
                self.set_frame_tree(&frame_tree, response_chan);
                self.send_viewport_rects();
                self.title_for_main_frame();
            }

            (Msg::ScrollFragmentPoint(pipeline_id, scroll_root_id, point, _),
             ShutdownState::NotShuttingDown) => {
                self.scroll_fragment_to_point(pipeline_id, scroll_root_id, point);
            }

            (Msg::MoveTo(point),
             ShutdownState::NotShuttingDown) => {
                self.window.set_position(point);
            }

            (Msg::ResizeTo(size),
             ShutdownState::NotShuttingDown) => {
                self.window.set_inner_size(size);
            }

            (Msg::GetClientWindow(send),
             ShutdownState::NotShuttingDown) => {
                let rect = self.window.client_window();
                if let Err(e) = send.send(rect) {
                    warn!("Sending response to get client window failed ({}).", e);
                }
            }

            (Msg::Status(message), ShutdownState::NotShuttingDown) => {
                self.window.status(message);
            }

            (Msg::LoadStart(back, forward), ShutdownState::NotShuttingDown) => {
                self.window.load_start(back, forward);
            }

            (Msg::LoadComplete(back, forward, root), ShutdownState::NotShuttingDown) => {
                self.got_load_complete_message = true;

                // If we're painting in headless mode, schedule a recomposite.
                if opts::get().output_file.is_some() || opts::get().exit_after_load {
                    self.composite_if_necessary(CompositingReason::Headless);
                }

                // Inform the embedder that the load has finished.
                //
                // TODO(pcwalton): Specify which frame's load completed.
                self.window.load_end(back, forward, root);
            }

            (Msg::DelayedCompositionTimeout(timestamp), ShutdownState::NotShuttingDown) => {
                if let CompositionRequest::DelayedComposite(this_timestamp) =
                    self.composition_request {
                    if timestamp == this_timestamp {
                        self.composition_request = CompositionRequest::CompositeNow(
                            CompositingReason::DelayedCompositeTimeout)
                    }
                }
            }

            (Msg::Recomposite(reason), ShutdownState::NotShuttingDown) => {
                self.composition_request = CompositionRequest::CompositeNow(reason)
            }

            (Msg::KeyEvent(ch, key, state, modified), ShutdownState::NotShuttingDown) => {
                if state == KeyState::Pressed {
                    self.window.handle_key(ch, key, modified);
                }
            }

            (Msg::TouchEventProcessed(result), ShutdownState::NotShuttingDown) => {
                self.touch_handler.on_event_processed(result);
            }

            (Msg::SetCursor(cursor), ShutdownState::NotShuttingDown) => {
                self.window.set_cursor(cursor)
            }

            (Msg::CreatePng(reply), ShutdownState::NotShuttingDown) => {
                let res = self.composite_specific_target(CompositeTarget::WindowAndPng);
                if let Err(ref e) = res {
                    info!("Error retrieving PNG: {:?}", e);
                }
                let img = res.unwrap_or(None);
                if let Err(e) = reply.send(img) {
                    warn!("Sending reply to create png failed ({}).", e);
                }
            }

            (Msg::ViewportConstrained(pipeline_id, constraints),
             ShutdownState::NotShuttingDown) => {
                self.constrain_viewport(pipeline_id, constraints);
            }

            (Msg::IsReadyToSaveImageReply(is_ready), ShutdownState::NotShuttingDown) => {
                assert!(self.ready_to_save_state == ReadyState::WaitingForConstellationReply);
                if is_ready {
                    self.ready_to_save_state = ReadyState::ReadyToSaveImage;
                    if opts::get().is_running_problem_test {
                        println!("ready to save image!");
                    }
                } else {
                    self.ready_to_save_state = ReadyState::Unknown;
                    if opts::get().is_running_problem_test {
                        println!("resetting ready_to_save_state!");
                    }
                }
                self.composite_if_necessary(CompositingReason::Headless);
            }

            (Msg::NewFavicon(url), ShutdownState::NotShuttingDown) => {
                self.window.set_favicon(url);
            }

            (Msg::HeadParsed, ShutdownState::NotShuttingDown) => {
                self.window.head_parsed();
            }

            (Msg::PipelineVisibilityChanged(pipeline_id, visible), ShutdownState::NotShuttingDown) => {
                self.pipeline_details(pipeline_id).visible = visible;
                if visible {
                    self.process_animations();
                }
            }

            (Msg::PipelineExited(pipeline_id, sender), _) => {
                debug!("Compositor got pipeline exited: {:?}", pipeline_id);
                self.remove_pipeline_root_layer(pipeline_id);
                let _ = sender.send(());
            }

            (Msg::NewScrollFrameReady(recomposite_needed), ShutdownState::NotShuttingDown) => {
                self.waiting_for_results_of_scroll = false;
                if recomposite_needed {
                    self.composition_request = CompositionRequest::CompositeNow(
                        CompositingReason::NewWebRenderScrollFrame);
                }
            }

            (Msg::Dispatch(func), ShutdownState::NotShuttingDown) => {
                // The functions sent here right now are really dumb, so they can't panic.
                // But if we start running more complex code here, we should really catch panic here.
                func();
            }

            (Msg::SetFullscreenState(state), ShutdownState::NotShuttingDown) => {
                self.window.set_fullscreen_state(state);
            }

            // When we are shutting_down, we need to avoid performing operations
            // such as Paint that may crash because we have begun tearing down
            // the rest of our resources.
            (_, ShutdownState::ShuttingDown) => { }
        }

        true
    }

    /// Sets or unsets the animations-running flag for the given pipeline, and schedules a
    /// recomposite if necessary.
    fn change_running_animations_state(&mut self,
                                       pipeline_id: PipelineId,
                                       animation_state: AnimationState) {
        match animation_state {
            AnimationState::AnimationsPresent => {
                let visible = self.pipeline_details(pipeline_id).visible;
                self.pipeline_details(pipeline_id).animations_running = true;
                if visible {
                    self.composite_if_necessary(CompositingReason::Animation);
                }
            }
            AnimationState::AnimationCallbacksPresent => {
                let visible = self.pipeline_details(pipeline_id).visible;
                self.pipeline_details(pipeline_id).animation_callbacks_running = true;
                if visible {
                    self.tick_animations_for_pipeline(pipeline_id);
                }
            }
            AnimationState::NoAnimationsPresent => {
                self.pipeline_details(pipeline_id).animations_running = false;
            }
            AnimationState::NoAnimationCallbacksPresent => {
                self.pipeline_details(pipeline_id).animation_callbacks_running = false;
            }
        }
    }

    fn pipeline_details(&mut self, pipeline_id: PipelineId) -> &mut PipelineDetails {
        if !self.pipeline_details.contains_key(&pipeline_id) {
            self.pipeline_details.insert(pipeline_id, PipelineDetails::new());
        }
        self.pipeline_details.get_mut(&pipeline_id).expect("Insert then get failed!")
    }

    pub fn pipeline(&self, pipeline_id: PipelineId) -> Option<&CompositionPipeline> {
        match self.pipeline_details.get(&pipeline_id) {
            Some(ref details) => details.pipeline.as_ref(),
            None => {
                warn!("Compositor layer has an unknown pipeline ({:?}).", pipeline_id);
                None
            }
        }
    }

    fn change_page_title(&mut self, pipeline_id: PipelineId, title: Option<String>) {
        let set_title = self.root_pipeline.as_ref().map_or(false, |root_pipeline| {
            root_pipeline.id == pipeline_id
        });
        if set_title {
            self.window.set_page_title(title);
        }
    }

    fn change_page_url(&mut self, _: PipelineId, url: ServoUrl) {
        self.window.set_page_url(url);
    }

    fn set_frame_tree(&mut self,
                      frame_tree: &SendableFrameTree,
                      response_chan: IpcSender<()>) {
        debug!("Setting the frame tree for pipeline {}", frame_tree.pipeline.id);
        if let Err(e) = response_chan.send(()) {
            warn!("Sending reponse to set frame tree failed ({}).", e);
        }

        self.root_pipeline = Some(frame_tree.pipeline.clone());

        let pipeline_id = frame_tree.pipeline.id.to_webrender();
        self.webrender_api.set_root_pipeline(pipeline_id);

        self.create_pipeline_details_for_frame_tree(&frame_tree);

        self.send_window_size(WindowSizeType::Initial);

        self.frame_tree_id.next();
    }

    fn create_pipeline_details_for_frame_tree(&mut self, frame_tree: &SendableFrameTree) {
        self.pipeline_details(frame_tree.pipeline.id).pipeline = Some(frame_tree.pipeline.clone());

        for kid in &frame_tree.children {
            self.create_pipeline_details_for_frame_tree(kid);
        }
    }

    fn remove_pipeline_root_layer(&mut self, pipeline_id: PipelineId) {
        self.pipeline_details.remove(&pipeline_id);
    }

    fn send_window_size(&self, size_type: WindowSizeType) {
        let dppx = self.page_zoom * self.device_pixels_per_screen_px();
        let initial_viewport = self.window_size.to_f32() / dppx;
        let visible_viewport = initial_viewport / self.viewport_zoom;
        let msg = ConstellationMsg::WindowSize(WindowSizeData {
            device_pixel_ratio: dppx,
            initial_viewport: initial_viewport,
            visible_viewport: visible_viewport,
        }, size_type);

        if let Err(e) = self.constellation_chan.send(msg) {
            warn!("Sending window resize to constellation failed ({}).", e);
        }
    }

    fn schedule_delayed_composite_if_necessary(&mut self) {
        match self.composition_request {
            CompositionRequest::CompositeNow(_) => return,
            CompositionRequest::DelayedComposite(_) |
            CompositionRequest::NoCompositingNecessary => {}
        }

        let timestamp = precise_time_ns();
        self.delayed_composition_timer.schedule_composite(timestamp);
        self.composition_request = CompositionRequest::DelayedComposite(timestamp);
    }

    fn scroll_fragment_to_point(&mut self,
                                pipeline_id: PipelineId,
                                scroll_root_id: ScrollRootId,
                                point: Point2D<f32>) {
        self.webrender_api.scroll_layers_with_scroll_root_id(
            LayoutPoint::from_untyped(&point),
            pipeline_id.to_webrender(),
            ServoScrollRootId(scroll_root_id.0));
    }

    fn handle_window_message(&mut self, event: WindowEvent) {
        match event {
            WindowEvent::Idle => {}

            WindowEvent::Refresh => {
                self.composite();
            }

            WindowEvent::InitializeCompositing => {
                self.initialize_compositing();
            }

            WindowEvent::Viewport(point, size) => {
              self.viewport = Some((point, size));
            }

            WindowEvent::Resize(size) => {
                self.on_resize_window_event(size);
            }

            WindowEvent::LoadUrl(url_string) => {
                self.on_load_url_window_event(url_string);
            }

            WindowEvent::MouseWindowEventClass(mouse_window_event) => {
                self.on_mouse_window_event_class(mouse_window_event);
            }

            WindowEvent::MouseWindowMoveEventClass(cursor) => {
                self.on_mouse_window_move_event_class(cursor);
            }

            WindowEvent::Touch(event_type, identifier, location) => {
                match event_type {
                    TouchEventType::Down => self.on_touch_down(identifier, location),
                    TouchEventType::Move => self.on_touch_move(identifier, location),
                    TouchEventType::Up => self.on_touch_up(identifier, location),
                    TouchEventType::Cancel => self.on_touch_cancel(identifier, location),
                }
            }

            WindowEvent::Scroll(delta, cursor, phase) => {
                match phase {
                    TouchEventType::Move => self.on_scroll_window_event(delta, cursor),
                    TouchEventType::Up | TouchEventType::Cancel => {
                        self.on_scroll_end_window_event(delta, cursor);
                    }
                    TouchEventType::Down => {
                        self.on_scroll_start_window_event(delta, cursor);
                    }
                }
            }

            WindowEvent::Zoom(magnification) => {
                self.on_zoom_window_event(magnification);
            }

            WindowEvent::ResetZoom => {
                self.on_zoom_reset_window_event();
            }

            WindowEvent::PinchZoom(magnification) => {
                self.on_pinch_zoom_window_event(magnification);
            }

            WindowEvent::Navigation(direction) => {
                self.on_navigation_window_event(direction);
            }

            WindowEvent::TouchpadPressure(cursor, pressure, stage) => {
                self.on_touchpad_pressure_event(cursor, pressure, stage);
            }

            WindowEvent::KeyEvent(ch, key, state, modifiers) => {
                self.on_key_event(ch, key, state, modifiers);
            }

            WindowEvent::Quit => {
                if self.shutdown_state == ShutdownState::NotShuttingDown {
                    debug!("Shutting down the constellation for WindowEvent::Quit");
                    self.start_shutting_down();
                }
            }

            WindowEvent::Reload => {
                let msg = ConstellationMsg::Reload;
                if let Err(e) = self.constellation_chan.send(msg) {
                    warn!("Sending reload to constellation failed ({}).", e);
                }
            }
        }
    }

    fn on_resize_window_event(&mut self, new_size: TypedSize2D<u32, DevicePixel>) {
        debug!("compositor resizing to {:?}", new_size.to_untyped());

        // A size change could also mean a resolution change.
        let new_scale_factor = self.window.scale_factor();
        if self.scale_factor != new_scale_factor {
            self.scale_factor = new_scale_factor;
            self.update_zoom_transform();
        }

        if self.window_size == new_size {
            return;
        }

        self.window_size = new_size;

        self.send_window_size(WindowSizeType::Resize);
    }

    fn on_load_url_window_event(&mut self, url_string: String) {
        debug!("osmain: loading URL `{}`", url_string);
        self.got_load_complete_message = false;
        match ServoUrl::parse(&url_string) {
            Ok(url) => {
                self.window.set_page_url(url.clone());
                let msg = match self.root_pipeline {
                    Some(ref pipeline) => ConstellationMsg::LoadUrl(pipeline.id, LoadData::new(url, None, None)),
                    None => ConstellationMsg::InitLoadUrl(url)
                };
                if let Err(e) = self.constellation_chan.send(msg) {
                    warn!("Sending load url to constellation failed ({}).", e);
                }
            },
            Err(e) => warn!("Parsing URL {} failed ({}).", url_string, e),
        }
    }

    fn on_mouse_window_event_class(&mut self, mouse_window_event: MouseWindowEvent) {
        if opts::get().convert_mouse_to_touch {
            match mouse_window_event {
                MouseWindowEvent::Click(_, _) => {}
                MouseWindowEvent::MouseDown(_, p) => self.on_touch_down(TouchId(0), p),
                MouseWindowEvent::MouseUp(_, p) => self.on_touch_up(TouchId(0), p),
            }
            return
        }

        self.dispatch_mouse_window_event_class(mouse_window_event);
    }

    fn dispatch_mouse_window_event_class(&mut self, mouse_window_event: MouseWindowEvent) {
        let point = match mouse_window_event {
            MouseWindowEvent::Click(_, p) => p,
            MouseWindowEvent::MouseDown(_, p) => p,
            MouseWindowEvent::MouseUp(_, p) => p,
        };

        let root_pipeline_id = match self.get_root_pipeline_id() {
            Some(root_pipeline_id) => root_pipeline_id,
            None => return,
        };

        if let Some(pipeline) = self.pipeline(root_pipeline_id) {
            let dppx = self.page_zoom * self.device_pixels_per_screen_px();
            let translated_point = (point / dppx).to_untyped();
            let event_to_send = match mouse_window_event {
                MouseWindowEvent::Click(button, _) => {
                    MouseButtonEvent(MouseEventType::Click, button, translated_point)
                }
                MouseWindowEvent::MouseDown(button, _) => {
                    MouseButtonEvent(MouseEventType::MouseDown, button, translated_point)
                }
                MouseWindowEvent::MouseUp(button, _) => {
                    MouseButtonEvent(MouseEventType::MouseUp, button, translated_point)
                }
            };
            let msg = ConstellationControlMsg::SendEvent(root_pipeline_id, event_to_send);
            if let Err(e) = pipeline.script_chan.send(msg) {
                warn!("Sending control event to script failed ({}).", e);
            }
        }
    }

    fn on_mouse_window_move_event_class(&mut self, cursor: TypedPoint2D<f32, DevicePixel>) {
        if opts::get().convert_mouse_to_touch {
            self.on_touch_move(TouchId(0), cursor);
            return
        }

        self.dispatch_mouse_window_move_event_class(cursor);
    }

    fn dispatch_mouse_window_move_event_class(&mut self, cursor: TypedPoint2D<f32, DevicePixel>) {
        let root_pipeline_id = match self.get_root_pipeline_id() {
            Some(root_pipeline_id) => root_pipeline_id,
            None => return,
        };
        if self.pipeline(root_pipeline_id).is_none() {
            return;
        }

        let dppx = self.page_zoom * self.device_pixels_per_screen_px();
        let event_to_send = MouseMoveEvent(Some((cursor / dppx).to_untyped()));
        let msg = ConstellationControlMsg::SendEvent(root_pipeline_id, event_to_send);
        if let Some(pipeline) = self.pipeline(root_pipeline_id) {
            if let Err(e) = pipeline.script_chan.send(msg) {
                warn!("Sending mouse control event to script failed ({}).", e);
            }
        }
    }

    fn send_event_to_root_pipeline(&self, event: CompositorEvent) {
        let root_pipeline_id = match self.get_root_pipeline_id() {
            Some(root_pipeline_id) => root_pipeline_id,
            None => return,
        };

        if let Some(pipeline) = self.pipeline(root_pipeline_id) {
            let msg = ConstellationControlMsg::SendEvent(root_pipeline_id, event);
            if let Err(e) = pipeline.script_chan.send(msg) {
                warn!("Sending control event to script failed ({}).", e);
            }
        }
    }

    fn on_touch_down(&mut self, identifier: TouchId, point: TypedPoint2D<f32, DevicePixel>) {
        self.touch_handler.on_touch_down(identifier, point);
        let dppx = self.page_zoom * self.device_pixels_per_screen_px();
        let translated_point = (point / dppx).to_untyped();
        self.send_event_to_root_pipeline(TouchEvent(TouchEventType::Down,
                                                    identifier,
                                                    translated_point));
    }

    fn on_touch_move(&mut self, identifier: TouchId, point: TypedPoint2D<f32, DevicePixel>) {
        match self.touch_handler.on_touch_move(identifier, point) {
            TouchAction::Scroll(delta) => {
                match point.cast() {
                    Some(point) => self.on_scroll_window_event(delta, point),
                    None => error!("Point cast failed."),
                }
            }
            TouchAction::Zoom(magnification, scroll_delta) => {
                let cursor = TypedPoint2D::new(-1, -1);  // Make sure this hits the base layer.
                self.pending_scroll_zoom_events.push(ScrollZoomEvent {
                    magnification: magnification,
                    delta: scroll_delta,
                    cursor: cursor,
                    phase: ScrollEventPhase::Move(true),
                    event_count: 1,
                });
            }
            TouchAction::DispatchEvent => {
                let dppx = self.page_zoom * self.device_pixels_per_screen_px();
                let translated_point = (point / dppx).to_untyped();
                self.send_event_to_root_pipeline(TouchEvent(TouchEventType::Move,
                                                            identifier,
                                                            translated_point));
            }
            _ => {}
        }
    }

    fn on_touch_up(&mut self, identifier: TouchId, point: TypedPoint2D<f32, DevicePixel>) {
        let dppx = self.page_zoom * self.device_pixels_per_screen_px();
        let translated_point = (point / dppx).to_untyped();
        self.send_event_to_root_pipeline(TouchEvent(TouchEventType::Up,
                                                    identifier,
                                                    translated_point));
        if let TouchAction::Click = self.touch_handler.on_touch_up(identifier, point) {
            self.simulate_mouse_click(point);
        }
    }

    fn on_touch_cancel(&mut self, identifier: TouchId, point: TypedPoint2D<f32, DevicePixel>) {
        // Send the event to script.
        self.touch_handler.on_touch_cancel(identifier, point);
        let dppx = self.page_zoom * self.device_pixels_per_screen_px();
        let translated_point = (point / dppx).to_untyped();
        self.send_event_to_root_pipeline(TouchEvent(TouchEventType::Cancel,
                                                    identifier,
                                                    translated_point));
    }

    fn on_touchpad_pressure_event(&self,
                                  point: TypedPoint2D<f32, DevicePixel>,
                                  pressure: f32,
                                  phase: TouchpadPressurePhase) {
        if let Some(true) = PREFS.get("dom.forcetouch.enabled").as_boolean() {
            let dppx = self.page_zoom * self.device_pixels_per_screen_px();
            let translated_point = (point / dppx).to_untyped();
            self.send_event_to_root_pipeline(TouchpadPressureEvent(translated_point,
                                                                   pressure,
                                                                   phase));
        }
    }

    /// http://w3c.github.io/touch-events/#mouse-events
    fn simulate_mouse_click(&mut self, p: TypedPoint2D<f32, DevicePixel>) {
        let button = MouseButton::Left;
        self.dispatch_mouse_window_move_event_class(p);
        self.dispatch_mouse_window_event_class(MouseWindowEvent::MouseDown(button, p));
        self.dispatch_mouse_window_event_class(MouseWindowEvent::MouseUp(button, p));
        self.dispatch_mouse_window_event_class(MouseWindowEvent::Click(button, p));
    }

    fn on_scroll_window_event(&mut self,
                              delta: TypedPoint2D<f32, DevicePixel>,
                              cursor: TypedPoint2D<i32, DevicePixel>) {
        let event_phase = match (self.scroll_in_progress, self.in_scroll_transaction) {
            (false, None) => ScrollEventPhase::Start,
            (false, Some(last_scroll)) if last_scroll.elapsed() > Duration::from_millis(80) =>
                ScrollEventPhase::Start,
            (_, _) => ScrollEventPhase::Move(self.scroll_in_progress),
        };
        self.in_scroll_transaction = Some(Instant::now());
        self.pending_scroll_zoom_events.push(ScrollZoomEvent {
            magnification: 1.0,
            delta: delta,
            cursor: cursor,
            phase: event_phase,
            event_count: 1,
        });
    }

    fn on_scroll_start_window_event(&mut self,
                                    delta: TypedPoint2D<f32, DevicePixel>,
                                    cursor: TypedPoint2D<i32, DevicePixel>) {
        self.scroll_in_progress = true;
        self.pending_scroll_zoom_events.push(ScrollZoomEvent {
            magnification: 1.0,
            delta: delta,
            cursor: cursor,
            phase: ScrollEventPhase::Start,
            event_count: 1,
        });
    }

    fn on_scroll_end_window_event(&mut self,
                                  delta: TypedPoint2D<f32, DevicePixel>,
                                  cursor: TypedPoint2D<i32, DevicePixel>) {
        self.scroll_in_progress = false;
        self.pending_scroll_zoom_events.push(ScrollZoomEvent {
            magnification: 1.0,
            delta: delta,
            cursor: cursor,
            phase: ScrollEventPhase::End,
            event_count: 1,
        });
    }

    fn process_pending_scroll_events(&mut self) {
        let had_events = self.pending_scroll_zoom_events.len() > 0;

        // Batch up all scroll events into one, or else we'll do way too much painting.
        let mut last_combined_event: Option<ScrollZoomEvent> = None;
        for scroll_event in self.pending_scroll_zoom_events.drain(..) {
            let this_delta = scroll_event.delta;
            let this_cursor = scroll_event.cursor;
            if let Some(combined_event) = last_combined_event {
                if combined_event.phase != scroll_event.phase {
                    let delta = (combined_event.delta / self.scale).to_untyped();
                    let cursor =
                        (combined_event.cursor.to_f32() / self.scale).to_untyped();
                    let delta = webrender_traits::LayerPoint::from_untyped(&delta);
                    let location = webrender_traits::ScrollLocation::Delta(delta);
                    let cursor = webrender_traits::WorldPoint::from_untyped(&cursor);
                    self.webrender_api.scroll(location, cursor, combined_event.phase);
                    last_combined_event = None
                }
            }

            match (&mut last_combined_event, scroll_event.phase) {
                (last_combined_event @ &mut None, _) => {
                    *last_combined_event = Some(ScrollZoomEvent {
                        magnification: scroll_event.magnification,
                        delta: this_delta,
                        cursor: this_cursor,
                        phase: scroll_event.phase,
                        event_count: 1,
                    })
                }
                (&mut Some(ref mut last_combined_event),
                 ScrollEventPhase::Move(false)) => {
                    // Mac OS X sometimes delivers scroll events out of vsync during a
                    // fling. This causes events to get bunched up occasionally, causing
                    // nasty-looking "pops". To mitigate this, during a fling we average
                    // deltas instead of summing them.
                    let old_event_count =
                        ScaleFactor::new(last_combined_event.event_count as f32);
                    last_combined_event.event_count += 1;
                    let new_event_count =
                        ScaleFactor::new(last_combined_event.event_count as f32);
                    last_combined_event.delta =
                        (last_combined_event.delta * old_event_count + this_delta) /
                        new_event_count;
                }
                (&mut Some(ref mut last_combined_event), _) => {
                    last_combined_event.delta = last_combined_event.delta + this_delta;
                    last_combined_event.event_count += 1
                }
            }
        }

        // TODO(gw): Support zoom (WR issue #28).
        if let Some(combined_event) = last_combined_event {
            let delta = (combined_event.delta / self.scale).to_untyped();
            let delta = webrender_traits::LayoutPoint::from_untyped(&delta);
            let cursor = (combined_event.cursor.to_f32() / self.scale).to_untyped();
            let location = webrender_traits::ScrollLocation::Delta(delta);
            let cursor = webrender_traits::WorldPoint::from_untyped(&cursor);
            self.webrender_api.scroll(location, cursor, combined_event.phase);
            self.waiting_for_results_of_scroll = true
        }

        if had_events {
            self.send_viewport_rects();
        }
    }

    /// If there are any animations running, dispatches appropriate messages to the constellation.
    fn process_animations(&mut self) {
        let mut pipeline_ids = vec![];
        for (pipeline_id, pipeline_details) in &self.pipeline_details {
            if (pipeline_details.animations_running ||
                pipeline_details.animation_callbacks_running) &&
               pipeline_details.visible {
                   pipeline_ids.push(*pipeline_id);
            }
        }
        for pipeline_id in &pipeline_ids {
            self.tick_animations_for_pipeline(*pipeline_id)
        }
    }

    fn tick_animations_for_pipeline(&mut self, pipeline_id: PipelineId) {
        self.schedule_delayed_composite_if_necessary();
        let animation_callbacks_running = self.pipeline_details(pipeline_id).animation_callbacks_running;
        if animation_callbacks_running {
            let msg = ConstellationMsg::TickAnimation(pipeline_id, AnimationTickType::Script);
            if let Err(e) = self.constellation_chan.send(msg) {
                warn!("Sending tick to constellation failed ({}).", e);
            }
        }

        // We may need to tick animations in layout. (See #12749.)
        let animations_running = self.pipeline_details(pipeline_id).animations_running;
        if animations_running {
            let msg = ConstellationMsg::TickAnimation(pipeline_id, AnimationTickType::Layout);
            if let Err(e) = self.constellation_chan.send(msg) {
                warn!("Sending tick to constellation failed ({}).", e);
            }
        }
    }

    fn constrain_viewport(&mut self, pipeline_id: PipelineId, constraints: ViewportConstraints) {
        let is_root = self.root_pipeline.as_ref().map_or(false, |root_pipeline| {
            root_pipeline.id == pipeline_id
        });

        if is_root {
            // TODO: actual viewport size

            self.viewport_zoom = constraints.initial_zoom;
            self.min_viewport_zoom = constraints.min_zoom;
            self.max_viewport_zoom = constraints.max_zoom;
            self.update_zoom_transform();
        }
    }

    fn device_pixels_per_screen_px(&self) -> ScaleFactor<f32, ScreenPx, DevicePixel> {
        match opts::get().device_pixels_per_px {
            Some(device_pixels_per_px) => ScaleFactor::new(device_pixels_per_px),
            None => match opts::get().output_file {
                Some(_) => ScaleFactor::new(1.0),
                None => self.scale_factor
            }
        }
    }

    fn device_pixels_per_page_px(&self) -> ScaleFactor<f32, PagePx, DevicePixel> {
        self.viewport_zoom * self.page_zoom * self.device_pixels_per_screen_px()
    }

    fn update_zoom_transform(&mut self) {
        let scale = self.device_pixels_per_page_px();
        self.scale = ScaleFactor::new(scale.get());
    }

    fn on_zoom_reset_window_event(&mut self) {
        self.page_zoom = ScaleFactor::new(1.0);
        self.update_zoom_transform();
        self.send_window_size(WindowSizeType::Resize);
    }

    fn on_zoom_window_event(&mut self, magnification: f32) {
        self.page_zoom = ScaleFactor::new((self.page_zoom.get() * magnification)
                                          .max(MIN_ZOOM).min(MAX_ZOOM));
        self.update_zoom_transform();
        self.send_window_size(WindowSizeType::Resize);
    }

    /// Simulate a pinch zoom
    fn on_pinch_zoom_window_event(&mut self, magnification: f32) {
        self.pending_scroll_zoom_events.push(ScrollZoomEvent {
            magnification: magnification,
            delta: TypedPoint2D::zero(), // TODO: Scroll to keep the center in view?
            cursor:  TypedPoint2D::new(-1, -1), // Make sure this hits the base layer.
            phase: ScrollEventPhase::Move(true),
            event_count: 1,
        });
    }

    fn on_navigation_window_event(&self, direction: WindowNavigateMsg) {
        let direction = match direction {
            windowing::WindowNavigateMsg::Forward => TraversalDirection::Forward(1),
            windowing::WindowNavigateMsg::Back => TraversalDirection::Back(1),
        };
        let msg = ConstellationMsg::TraverseHistory(None, direction);
        if let Err(e) = self.constellation_chan.send(msg) {
            warn!("Sending navigation to constellation failed ({}).", e);
        }
    }

    fn on_key_event(&mut self,
                    ch: Option<char>,
                    key: Key,
                    state: KeyState,
                    modifiers: KeyModifiers) {
        // Steal a few key events for webrender debug options.
        if modifiers.contains(CONTROL) && state == KeyState::Pressed {
            match key {
                Key::F12 => {
                    let profiler_enabled = self.webrender.get_profiler_enabled();
                    self.webrender.set_profiler_enabled(!profiler_enabled);
                    return;
                }
                _ => {}
            }
        }

        let msg = ConstellationMsg::KeyEvent(ch, key, state, modifiers);
        if let Err(e) = self.constellation_chan.send(msg) {
            warn!("Sending key event to constellation failed ({}).", e);
        }
    }

    fn send_viewport_rects(&self) {
        let mut stacking_context_scroll_states_per_pipeline = HashMap::new();
        for scroll_layer_state in self.webrender_api.get_scroll_layer_state() {
            let stacking_context_scroll_state = StackingContextScrollState {
                scroll_root_id: scroll_layer_state.scroll_root_id.from_webrender(),
                scroll_offset: scroll_layer_state.scroll_offset.to_untyped(),
            };
            let pipeline_id = scroll_layer_state.pipeline_id;
            stacking_context_scroll_states_per_pipeline
                .entry(pipeline_id)
                .or_insert(vec![])
                .push(stacking_context_scroll_state);
        }

        for (pipeline_id, stacking_context_scroll_states) in
                stacking_context_scroll_states_per_pipeline {
            if let Some(pipeline) = self.pipeline(pipeline_id.from_webrender()) {
                let msg = LayoutControlMsg::SetStackingContextScrollStates(
                    stacking_context_scroll_states);
                let _ = pipeline.layout_chan.send(msg);
            }
        }
    }

    // Check if any pipelines currently have active animations or animation callbacks.
    fn animations_active(&self) -> bool {
        for (_, details) in &self.pipeline_details {
            // If animations are currently running, then don't bother checking
            // with the constellation if the output image is stable.
            if details.animations_running {
                return true;
            }
            if details.animation_callbacks_running {
                return true;
            }
        }

        false
    }

    /// Query the constellation to see if the current compositor
    /// output matches the current frame tree output, and if the
    /// associated script threads are idle.
    fn is_ready_to_paint_image_output(&mut self) -> Result<(), NotReadyToPaint> {
        match self.ready_to_save_state {
            ReadyState::Unknown => {
                // Unsure if the output image is stable.

                // Collect the currently painted epoch of each pipeline that is
                // complete (i.e. has *all* layers painted to the requested epoch).
                // This gets sent to the constellation for comparison with the current
                // frame tree.
                let mut pipeline_epochs = HashMap::new();
                for (id, _) in &self.pipeline_details {
                    let webrender_pipeline_id = id.to_webrender();
                    if let Some(webrender_traits::Epoch(epoch)) = self.webrender
                                                                      .current_epoch(webrender_pipeline_id) {
                        let epoch = Epoch(epoch);
                        pipeline_epochs.insert(*id, epoch);
                    }
                }

                // Pass the pipeline/epoch states to the constellation and check
                // if it's safe to output the image.
                let msg = ConstellationMsg::IsReadyToSaveImage(pipeline_epochs);
                if let Err(e) = self.constellation_chan.send(msg) {
                    warn!("Sending ready to save to constellation failed ({}).", e);
                }
                self.ready_to_save_state = ReadyState::WaitingForConstellationReply;
                Err(NotReadyToPaint::JustNotifiedConstellation)
            }
            ReadyState::WaitingForConstellationReply => {
                // If waiting on a reply from the constellation to the last
                // query if the image is stable, then assume not ready yet.
                Err(NotReadyToPaint::WaitingOnConstellation)
            }
            ReadyState::ReadyToSaveImage => {
                // Constellation has replied at some point in the past
                // that the current output image is stable and ready
                // for saving.
                // Reset the flag so that we check again in the future
                // TODO: only reset this if we load a new document?
                if opts::get().is_running_problem_test {
                    println!("was ready to save, resetting ready_to_save_state");
                }
                self.ready_to_save_state = ReadyState::Unknown;
                Ok(())
            }
        }
    }

    fn composite(&mut self) {
        let target = self.composite_target;
        match self.composite_specific_target(target) {
            Ok(_) => if opts::get().output_file.is_some() || opts::get().exit_after_load {
                println!("Shutting down the Constellation after generating an output file or exit flag specified");
                self.start_shutting_down();
            },
            Err(e) => if opts::get().is_running_problem_test {
                if e != UnableToComposite::NotReadyToPaintImage(NotReadyToPaint::WaitingOnConstellation) {
                    println!("not ready to composite: {:?}", e);
                }
            },
        }
    }

    /// Composite either to the screen or to a png image or both.
    /// Returns Ok if composition was performed or Err if it was not possible to composite
    /// for some reason. If CompositeTarget is Window or Png no image data is returned;
    /// in the latter case the image is written directly to a file. If CompositeTarget
    /// is WindowAndPng Ok(Some(png::Image)) is returned.
    fn composite_specific_target(&mut self,
                                 target: CompositeTarget)
                                 -> Result<Option<Image>, UnableToComposite> {
        let (width, height) =
            (self.window_size.width as usize, self.window_size.height as usize);
        if !self.window.prepare_for_composite(width, height) {
            return Err(UnableToComposite::WindowUnprepared)
        }

        self.webrender.update();

        let wait_for_stable_image = match target {
            CompositeTarget::WindowAndPng | CompositeTarget::PngFile => true,
            CompositeTarget::Window => opts::get().exit_after_load,
        };

        if wait_for_stable_image {
            match self.is_ready_to_paint_image_output() {
                Ok(()) => {
                    // The current image is ready to output. However, if there are animations active,
                    // tick those instead and continue waiting for the image output to be stable AND
                    // all active animations to complete.
                    if self.animations_active() {
                        self.process_animations();
                        return Err(UnableToComposite::NotReadyToPaintImage(NotReadyToPaint::AnimationsActive));
                    }
                }
                Err(result) => {
                    return Err(UnableToComposite::NotReadyToPaintImage(result))
                }
            }
        }

        let render_target_info = match target {
            CompositeTarget::Window => RenderTargetInfo::empty(),
            _ => initialize_png(width, height)
        };

        profile(ProfilerCategory::Compositing, None, self.time_profiler_chan.clone(), || {
            debug!("compositor: compositing");

            // Paint the scene.
            let size = webrender_traits::DeviceUintSize::from_untyped(&self.window_size.to_untyped());
            self.webrender.render(size);
        });

        let rv = match target {
            CompositeTarget::Window => None,
            CompositeTarget::WindowAndPng => {
                let img = self.draw_img(render_target_info,
                                        width,
                                        height);
                Some(Image {
                    width: img.width(),
                    height: img.height(),
                    format: PixelFormat::RGB8,
                    bytes: IpcSharedMemory::from_bytes(&*img),
                    id: None,
                })
            }
            CompositeTarget::PngFile => {
                profile(ProfilerCategory::ImageSaving, None, self.time_profiler_chan.clone(), || {
                    match opts::get().output_file.as_ref() {
                        Some(path) => match File::create(path) {
                            Ok(mut file) => {
                                let img = self.draw_img(render_target_info, width, height);
                                let dynamic_image = DynamicImage::ImageRgb8(img);
                                if let Err(e) = dynamic_image.save(&mut file, ImageFormat::PNG) {
                                    error!("Failed to save {} ({}).", path, e);
                                }
                            },
                            Err(e) => error!("Failed to create {} ({}).", path, e),
                        },
                        None => error!("No file specified."),
                    }
                });
                None
            }
        };

        // Perform the page flip. This will likely block for a while.
        self.window.present();

        self.last_composite_time = precise_time_ns();

        self.composition_request = CompositionRequest::NoCompositingNecessary;

        self.process_animations();
        self.start_scrolling_bounce_if_necessary();
        self.waiting_for_results_of_scroll = false;

        Ok(rv)
    }

    fn draw_img(&self,
                render_target_info: RenderTargetInfo,
                width: usize,
                height: usize)
                -> RgbImage {
        let mut pixels = gl::read_pixels(0, 0,
                                         width as gl::GLsizei,
                                         height as gl::GLsizei,
                                         gl::RGB, gl::UNSIGNED_BYTE);

        gl::bind_framebuffer(gl::FRAMEBUFFER, 0);

        gl::delete_buffers(&render_target_info.texture_ids);
        gl::delete_renderbuffers(&render_target_info.renderbuffer_ids);
        gl::delete_frame_buffers(&render_target_info.framebuffer_ids);

        // flip image vertically (texture is upside down)
        let orig_pixels = pixels.clone();
        let stride = width * 3;
        for y in 0..height {
            let dst_start = y * stride;
            let src_start = (height - y - 1) * stride;
            let src_slice = &orig_pixels[src_start .. src_start + stride];
            (&mut pixels[dst_start .. dst_start + stride]).clone_from_slice(&src_slice[..stride]);
        }
        RgbImage::from_raw(width as u32, height as u32, pixels).expect("Flipping image failed!")
    }

    fn composite_if_necessary(&mut self, reason: CompositingReason) {
        if self.composition_request == CompositionRequest::NoCompositingNecessary {
            if opts::get().is_running_problem_test {
                println!("updating composition_request ({:?})", reason);
            }
            self.composition_request = CompositionRequest::CompositeNow(reason)
        } else if opts::get().is_running_problem_test {
            println!("composition_request is already {:?}", self.composition_request);
        }
    }

    fn initialize_compositing(&mut self) {
    }

    fn get_root_pipeline_id(&self) -> Option<PipelineId> {
        self.root_pipeline.as_ref().map(|pipeline| pipeline.id)
    }

    fn start_scrolling_bounce_if_necessary(&mut self) {
        if self.scroll_in_progress {
            return
        }

        if self.webrender.layers_are_bouncing_back() {
            self.webrender_api.tick_scrolling_bounce_animations();
            self.send_viewport_rects()
        }
    }

    pub fn handle_events(&mut self, messages: Vec<WindowEvent>) -> bool {
        // Check for new messages coming from the other threads in the system.
        let mut compositor_messages = vec![];
        let mut found_recomposite_msg = false;
        while let Some(msg) = self.port.try_recv_compositor_msg() {
            match msg {
                Msg::Recomposite(_) if found_recomposite_msg => {}
                Msg::Recomposite(_) => {
                    found_recomposite_msg = true;
                    compositor_messages.push(msg)
                }
                _ => compositor_messages.push(msg),
            }
        }
        if found_recomposite_msg {
            compositor_messages.retain(|msg| {
                match *msg {
                    Msg::DelayedCompositionTimeout(_) => false,
                    _ => true,
                }
            })
        }
        for msg in compositor_messages {
            if !self.handle_browser_message(msg) {
                break
            }
        }

        if self.shutdown_state == ShutdownState::FinishedShuttingDown {
            return false;
        }

        // Handle any messages coming from the windowing system.
        for message in messages {
            self.handle_window_message(message);
        }

        // If a pinch-zoom happened recently, ask for tiles at the new resolution
        if self.zoom_action && precise_time_s() - self.zoom_time > 0.3 {
            self.zoom_action = false;
        }

        match self.composition_request {
            CompositionRequest::NoCompositingNecessary |
            CompositionRequest::DelayedComposite(_) => {}
            CompositionRequest::CompositeNow(_) => {
                self.composite()
            }
        }

        if !self.pending_scroll_zoom_events.is_empty() && !self.waiting_for_results_of_scroll {
            self.process_pending_scroll_events()
        }

        self.shutdown_state != ShutdownState::FinishedShuttingDown
    }

    /// Repaints and recomposites synchronously. You must be careful when calling this, as if a
    /// paint is not scheduled the compositor will hang forever.
    ///
    /// This is used when resizing the window.
    pub fn repaint_synchronously(&mut self) {
        while self.shutdown_state != ShutdownState::ShuttingDown {
            let msg = self.port.recv_compositor_msg();
            let need_recomposite = match msg {
                Msg::Recomposite(_) => true,
                _ => false,
            };
            let keep_going = self.handle_browser_message(msg);
            if need_recomposite {
                self.composite();
                break
            }
            if !keep_going {
                break
            }
        }
    }

    pub fn pinch_zoom_level(&self) -> f32 {
        self.viewport_zoom.get() as f32
    }

    pub fn title_for_main_frame(&self) {
        let root_pipeline_id = match self.root_pipeline {
            None => return,
            Some(ref root_pipeline) => root_pipeline.id,
        };
        let msg = ConstellationMsg::GetPipelineTitle(root_pipeline_id);
        if let Err(e) = self.constellation_chan.send(msg) {
            warn!("Failed to send pipeline title ({}).", e);
        }
    }
}

/// Why we performed a composite. This is used for debugging.
#[derive(Copy, Clone, PartialEq, Debug)]
pub enum CompositingReason {
    /// We hit the delayed composition timeout. (See `delayed_composition.rs`.)
    DelayedCompositeTimeout,
    /// The window has been scrolled and we're starting the first recomposite.
    Scroll,
    /// A scroll has continued and we need to recomposite again.
    ContinueScroll,
    /// We're performing the single composite in headless mode.
    Headless,
    /// We're performing a composite to run an animation.
    Animation,
    /// A new frame tree has been loaded.
    NewFrameTree,
    /// New painted buffers have been received.
    NewPaintedBuffers,
    /// The window has been zoomed.
    Zoom,
    /// A new WebRender frame has arrived.
    NewWebRenderFrame,
    /// WebRender has processed a scroll event and has generated a new frame.
    NewWebRenderScrollFrame,
}