libservo: Move animation tracking from `WindowMethods` to delegates (#36400)

This changes removes animation tracking from the `WindowMethods` trait
and moves it to `ServoDelegate` and `WebViewDelegate`.

- Animation changes per-`WebView` are now triggered in the compositor
  only when the value is updated there, rather than right after ticking
  animations.
- Both `WebView` and `Servo` now expose an `animation()` method, so
  tracking animation state actually becomes unecessary in many cases,
  such as that of desktop servoshell, which can just read the value
  when the event loop spins.

Testing: No tests necessary as the API layer is still untested. Later,
tests will be added for the `WebView` API and this can be tested then.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

15 files changed, 153 insertions, 125 deletions

diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index d4c522aa05e..ccb28c5df56 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -58,7 +58,7 @@ use webrender_api::{
 use crate::InitialCompositorState;
 use crate::webview::{UnknownWebView, WebView};
 use crate::webview_manager::WebViewManager;
-use crate::windowing::{self, WebRenderDebugOption, WindowMethods};
+use crate::windowing::{WebRenderDebugOption, WindowMethods};
 
 #[derive(Debug, PartialEq)]
 enum UnableToComposite {
@@ -258,22 +258,20 @@ impl PipelineDetails {
         self.animation_callbacks_running
     }
 
-    pub(crate) fn tick_animations(&self, compositor: &IOCompositor) -> bool {
-        let animation_callbacks_running = self.animation_callbacks_running;
-        let animations_running = self.animations_running;
-        if !animation_callbacks_running && !animations_running {
-            return false;
-        }
+    pub(crate) fn animating(&self) -> bool {
+        !self.throttled && (self.animation_callbacks_running || self.animations_running)
+    }
 
-        if self.throttled {
-            return false;
+    pub(crate) fn tick_animations(&self, compositor: &IOCompositor) {
+        if !self.animating() {
+            return;
         }
 
         let mut tick_type = AnimationTickType::empty();
-        if animations_running {
+        if self.animations_running {
             tick_type.insert(AnimationTickType::CSS_ANIMATIONS_AND_TRANSITIONS);
         }
-        if animation_callbacks_running {
+        if self.animation_callbacks_running {
             tick_type.insert(AnimationTickType::REQUEST_ANIMATION_FRAME);
         }
 
@@ -281,7 +279,6 @@ impl PipelineDetails {
         if let Err(e) = compositor.global.borrow().constellation_sender.send(msg) {
             warn!("Sending tick to constellation failed ({:?}).", e);
         }
-        true
     }
 }
 
@@ -490,6 +487,17 @@ impl IOCompositor {
         }
     }
 
+    pub fn webxr_running(&self) -> bool {
+        #[cfg(feature = "webxr")]
+        {
+            self.global.borrow().webxr_main_thread.running()
+        }
+        #[cfg(not(feature = "webxr"))]
+        {
+            false
+        }
+    }
+
     fn set_needs_repaint(&self, reason: RepaintReason) {
         let mut needs_repaint = self.needs_repaint.get();
         needs_repaint.insert(reason);
@@ -1313,23 +1321,9 @@ impl IOCompositor {
         }
         self.last_animation_tick = Instant::now();
 
-        #[cfg(feature = "webxr")]
-        let webxr_running = self.global.borrow().webxr_main_thread.running();
-        #[cfg(not(feature = "webxr"))]
-        let webxr_running = false;
-
-        let any_webviews_animating = !self
-            .webviews
-            .iter()
-            .all(|webview| !webview.tick_all_animations(self));
-
-        let animation_state = if !any_webviews_animating && !webxr_running {
-            windowing::AnimationState::Idle
-        } else {
-            windowing::AnimationState::Animating
-        };
-
-        self.window.set_animation_state(animation_state);
+        for webview in self.webviews.iter() {
+            webview.tick_all_animations(self);
+        }
     }
 
     pub(crate) fn device_pixels_per_page_pixel(&self) -> Scale<f32, CSSPixel, DevicePixel> {
diff --git a/components/compositing/webview.rs b/components/compositing/webview.rs
index 9e8136f28ad..5d365f3b89b 100644
--- a/components/compositing/webview.rs
+++ b/components/compositing/webview.rs
@@ -217,37 +217,41 @@ impl WebView {
             })
     }
 
-    /// Sets or unsets the animations-running flag for the given pipeline, and schedules a
-    /// recomposite if necessary. Returns true if the pipeline is throttled.
+    /// Sets or unsets the animations-running flag for the given pipeline. Returns true if
+    /// the pipeline is throttled.
     pub(crate) fn change_running_animations_state(
         &mut self,
         pipeline_id: PipelineId,
         animation_state: AnimationState,
     ) -> bool {
-        let pipeline_details = self.ensure_pipeline_details(pipeline_id);
-        match animation_state {
-            AnimationState::AnimationsPresent => {
-                pipeline_details.animations_running = true;
-            },
-            AnimationState::AnimationCallbacksPresent => {
-                pipeline_details.animation_callbacks_running = true;
-            },
-            AnimationState::NoAnimationsPresent => {
-                pipeline_details.animations_running = false;
-            },
-            AnimationState::NoAnimationCallbacksPresent => {
-                pipeline_details.animation_callbacks_running = false;
-            },
-        }
-        pipeline_details.throttled
+        let throttled = {
+            let pipeline_details = self.ensure_pipeline_details(pipeline_id);
+            match animation_state {
+                AnimationState::AnimationsPresent => {
+                    pipeline_details.animations_running = true;
+                },
+                AnimationState::AnimationCallbacksPresent => {
+                    pipeline_details.animation_callbacks_running = true;
+                },
+                AnimationState::NoAnimationsPresent => {
+                    pipeline_details.animations_running = false;
+                },
+                AnimationState::NoAnimationCallbacksPresent => {
+                    pipeline_details.animation_callbacks_running = false;
+                },
+            }
+            pipeline_details.throttled
+        };
+
+        let animating = self.pipelines.values().any(PipelineDetails::animating);
+        self.renderer_webview.set_animating(animating);
+        throttled
     }
 
-    pub(crate) fn tick_all_animations(&self, compositor: &IOCompositor) -> bool {
-        let mut ticked_any = false;
+    pub(crate) fn tick_all_animations(&self, compositor: &IOCompositor) {
         for pipeline_details in self.pipelines.values() {
-            ticked_any = pipeline_details.tick_animations(compositor) || ticked_any;
+            pipeline_details.tick_animations(compositor)
         }
-        ticked_any
     }
 
     pub(crate) fn tick_animations_for_pipeline(
diff --git a/components/compositing/windowing.rs b/components/compositing/windowing.rs
index a73e65f11d1..1ab30e73154 100644
--- a/components/compositing/windowing.rs
+++ b/components/compositing/windowing.rs
@@ -4,8 +4,6 @@
 
 //! Abstract windowing methods. The concrete implementations of these can be found in `platform/`.
 
-use std::fmt::Debug;
-
 use embedder_traits::{EventLoopWaker, MouseButton};
 use euclid::Scale;
 use net::protocols::ProtocolRegistry;
@@ -27,12 +25,6 @@ pub enum WebRenderDebugOption {
     RenderTargetDebug,
 }
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-pub enum AnimationState {
-    Idle,
-    Animating,
-}
-
 // TODO: this trait assumes that the window is responsible
 // for creating the GL context, making it current, buffer
 // swapping, etc. Really that should all be done by surfman.
@@ -40,12 +32,6 @@ pub trait WindowMethods {
     /// Get the HighDPI factor of the native window, the screen and the framebuffer.
     /// TODO(martin): Move this to `RendererWebView` when possible.
     fn hidpi_factor(&self) -> Scale<f32, DeviceIndependentPixel, DevicePixel>;
-    /// Set whether the application is currently animating.
-    /// Typically, when animations are active, the window
-    /// will want to avoid blocking on UI events, and just
-    /// run the event loop at the vsync interval.
-    /// TODO(martin): Move this to `RendererWebView` when possible.
-    fn set_animation_state(&self, _state: AnimationState);
 }
 
 pub trait EmbedderMethods {
diff --git a/components/servo/examples/winit_minimal.rs b/components/servo/examples/winit_minimal.rs
index f19fd9763fe..1f6dbbe5c12 100644
--- a/components/servo/examples/winit_minimal.rs
+++ b/components/servo/examples/winit_minimal.rs
@@ -1,11 +1,11 @@
 /* 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 https://mozilla.org/MPL/2.0/. */
 
-use std::cell::{Cell, RefCell};
+use std::cell::RefCell;
 use std::error::Error;
 use std::rc::Rc;
 
-use compositing::windowing::{AnimationState, EmbedderMethods, WindowMethods};
+use compositing::windowing::{EmbedderMethods, WindowMethods};
 use euclid::{Point2D, Scale, Size2D};
 use servo::{RenderingContext, Servo, TouchEventType, WebView, WindowRenderingContext};
 use servo_geometry::DeviceIndependentPixel;
@@ -236,15 +236,11 @@ impl embedder_traits::EventLoopWaker for Waker {
 
 struct WindowDelegate {
     window: Window,
-    animation_state: Cell<AnimationState>,
 }
 
 impl WindowDelegate {
     fn new(window: Window) -> Self {
-        Self {
-            window,
-            animation_state: Cell::new(AnimationState::Idle),
-        }
+        Self { window }
     }
 }
 
@@ -252,10 +248,6 @@ impl WindowMethods for WindowDelegate {
     fn hidpi_factor(&self) -> Scale<f32, DeviceIndependentPixel, DevicePixel> {
         Scale::new(self.window.scale_factor() as f32)
     }
-
-    fn set_animation_state(&self, state: compositing::windowing::AnimationState) {
-        self.animation_state.set(state);
-    }
 }
 
 pub fn winit_size_to_euclid_size<T>(size: PhysicalSize<T>) -> Size2D<T, DevicePixel> {
diff --git a/components/servo/lib.rs b/components/servo/lib.rs
index 3ef27875d3a..eea6ea01392 100644
--- a/components/servo/lib.rs
+++ b/components/servo/lib.rs
@@ -210,6 +210,8 @@ pub struct Servo {
     /// and deinitialization of the JS Engine. Multiprocess Servo instances have their
     /// own instance that exists in the content process instead.
     _js_engine_setup: Option<JSEngineSetup>,
+    /// Whether or not any WebView in this instance is animating or WebXR is enabled.
+    animating: Cell<bool>,
 }
 
 #[derive(Clone)]
@@ -507,6 +509,7 @@ impl Servo {
             webviews: Default::default(),
             servo_errors: ServoErrorChannel::default(),
             _js_engine_setup: js_engine_setup,
+            animating: Cell::new(false),
         }
     }
 
@@ -518,6 +521,15 @@ impl Servo {
         *self.delegate.borrow_mut() = delegate;
     }
 
+    /// Whether or not any [`WebView`] of this Servo instance has animating content, such as a CSS
+    /// animation or transition or is running `requestAnimationFrame` callbacks. In addition, this
+    /// returns true if WebXR content is running. This indicates that the embedding application
+    /// should be spinning the Servo event loop on regular intervals in order to trigger animation
+    /// updates.
+    pub fn animating(&self) -> bool {
+        self.animating.get()
+    }
+
     /// **EXPERIMENTAL:** Intialize GL accelerated media playback. This currently only works on a limited number
     /// of platforms. This should be run *before* calling [`Servo::new`] and creating the first [`WebView`].
     pub fn initialize_gl_accelerated_media(display: NativeDisplay, api: GlApi, context: GlContext) {
@@ -555,6 +567,7 @@ impl Servo {
 
         self.compositor.borrow_mut().perform_updates();
         self.send_new_frame_ready_messages();
+        self.send_animating_changed_messages();
         self.handle_delegate_errors();
         self.clean_up_destroyed_webview_handles();
 
@@ -580,6 +593,19 @@ impl Servo {
         }
     }
 
+    fn send_animating_changed_messages(&self) {
+        let animating = self.compositor.borrow().webxr_running() ||
+            self.webviews
+                .borrow()
+                .values()
+                .filter_map(WebView::from_weak_handle)
+                .any(|webview| webview.animating());
+        if animating != self.animating.get() {
+            self.animating.set(animating);
+            self.delegate().notify_animating_changed(animating);
+        }
+    }
+
     fn handle_delegate_errors(&self) {
         while let Some(error) = self.servo_errors.try_recv() {
             self.delegate().notify_error(self, error);
diff --git a/components/servo/servo_delegate.rs b/components/servo/servo_delegate.rs
index 8ec53a4790b..b119a962029 100644
--- a/components/servo/servo_delegate.rs
+++ b/components/servo/servo_delegate.rs
@@ -28,6 +28,11 @@ pub trait ServoDelegate {
     /// Request a DevTools connection from a DevTools client. Typically an embedder application
     /// will show a permissions prompt when this happens to confirm a connection is allowed.
     fn request_devtools_connection(&self, _servo: &Servo, _request: AllowOrDenyRequest) {}
+    /// Any [`WebView`] in this Servo instance has either started to animate or WebXR is
+    /// running. When a [`WebView`] is animating, it is up to the embedding application
+    /// ensure that `Servo::spin_event_loop` is called at regular intervals in order to
+    /// update the painted contents of the [`WebView`].
+    fn notify_animating_changed(&self, _animating: bool) {}
     /// Triggered when Servo will load a web (HTTP/HTTPS) resource. The load may be
     /// intercepted and alternate contents can be loaded by the client by calling
     /// [`WebResourceLoad::intercept`]. If not handled, the load will continue as normal.
diff --git a/components/servo/webview.rs b/components/servo/webview.rs
index ed243d56421..92564753168 100644
--- a/components/servo/webview.rs
+++ b/components/servo/webview.rs
@@ -81,6 +81,7 @@ pub(crate) struct WebViewInner {
     page_title: Option<String>,
     favicon_url: Option<Url>,
     focused: bool,
+    animating: bool,
     cursor: Cursor,
 }
 
@@ -110,6 +111,7 @@ impl WebView {
             page_title: None,
             favicon_url: None,
             focused: false,
+            animating: false,
             cursor: Cursor::Pointer,
         })));
 
@@ -265,6 +267,22 @@ impl WebView {
             .send(EmbedderToConstellationMessage::BlurWebView);
     }
 
+    /// Whether or not this [`WebView`] has animating content, such as a CSS animation or
+    /// transition or is running `requestAnimationFrame` callbacks. This indicates that the
+    /// embedding application should be spinning the Servo event loop on regular intervals
+    /// in order to trigger animation updates.
+    pub fn animating(self) -> bool {
+        self.inner().animating
+    }
+
+    pub(crate) fn set_animating(self, new_value: bool) {
+        if self.inner().animating == new_value {
+            return;
+        }
+        self.inner_mut().animating = new_value;
+        self.delegate().notify_animating_changed(self, new_value);
+    }
+
     pub fn rect(&self) -> DeviceRect {
         self.inner().rect
     }
@@ -475,12 +493,18 @@ struct ServoRendererWebView {
 }
 
 impl RendererWebView for ServoRendererWebView {
+    fn id(&self) -> WebViewId {
+        self.id
+    }
+
     fn screen_geometry(&self) -> Option<ScreenGeometry> {
         let webview = WebView::from_weak_handle(&self.weak_handle)?;
         webview.delegate().screen_geometry(webview)
     }
 
-    fn id(&self) -> WebViewId {
-        self.id
+    fn set_animating(&self, new_value: bool) {
+        if let Some(webview) = WebView::from_weak_handle(&self.weak_handle) {
+            webview.set_animating(new_value);
+        }
     }
 }
diff --git a/components/servo/webview_delegate.rs b/components/servo/webview_delegate.rs
index b631eae7404..ac3dc7a5152 100644
--- a/components/servo/webview_delegate.rs
+++ b/components/servo/webview_delegate.rs
@@ -375,6 +375,11 @@ pub trait WebViewDelegate {
     /// This [`WebView`] has either become focused or lost focus. Whether or not the
     /// [`WebView`] is focused can be accessed via [`WebView::focused`].
     fn notify_focus_changed(&self, _webview: WebView, _focused: bool) {}
+    /// This [`WebView`] has either started to animate or stopped animating. When a
+    /// [`WebView`] is animating, it is up to the embedding application ensure that
+    /// `Servo::spin_event_loop` is called at regular intervals in order to update the
+    /// painted contents of the [`WebView`].
+    fn notify_animating_changed(&self, _webview: WebView, _animating: bool) {}
     /// The `LoadStatus` of the currently loading or loaded page in this [`WebView`] has changed. The new
     /// status can accessed via [`WebView::load_status`].
     fn notify_load_status_changed(&self, _webview: WebView, _status: LoadStatus) {}
diff --git a/components/shared/compositing/lib.rs b/components/shared/compositing/lib.rs
index c231960ef50..baf59e53118 100644
--- a/components/shared/compositing/lib.rs
+++ b/components/shared/compositing/lib.rs
@@ -590,4 +590,5 @@ impl From<SerializableImageData> for ImageData {
 pub trait RendererWebView {
     fn id(&self) -> WebViewId;
     fn screen_geometry(&self) -> Option<ScreenGeometry>;
+    fn set_animating(&self, new_value: bool);
 }
diff --git a/ports/servoshell/desktop/app.rs b/ports/servoshell/desktop/app.rs
index 047197a60a8..657228e9d44 100644
--- a/ports/servoshell/desktop/app.rs
+++ b/ports/servoshell/desktop/app.rs
@@ -13,7 +13,7 @@ use std::{env, fs};
 
 use euclid::Scale;
 use log::{info, trace, warn};
-use servo::compositing::windowing::{AnimationState, WindowMethods};
+use servo::compositing::windowing::WindowMethods;
 use servo::config::opts::Opts;
 use servo::config::prefs::Preferences;
 use servo::servo_config::pref;
@@ -146,9 +146,6 @@ impl App {
             fn hidpi_factor(&self) -> Scale<f32, DeviceIndependentPixel, DevicePixel> {
                 self.0.hidpi_factor()
             }
-            fn set_animation_state(&self, state: AnimationState) {
-                self.0.set_animation_state(state);
-            }
         }
 
         let mut user_content_manager = UserContentManager::new();
@@ -183,8 +180,12 @@ impl App {
         self.state = AppState::Running(running_state);
     }
 
-    pub fn is_animating(&self) -> bool {
-        self.windows.iter().any(|(_, window)| window.is_animating())
+    pub(crate) fn animating(&self) -> bool {
+        match self.state {
+            AppState::Initializing => false,
+            AppState::Running(ref running_app_state) => running_app_state.servo().animating(),
+            AppState::ShuttingDown => false,
+        }
     }
 
     /// Handle events with winit contexts
@@ -401,10 +402,8 @@ impl ApplicationHandler<WakerEvent> for App {
             window.handle_winit_event(state.clone(), event);
         }
 
-        let animating = self.is_animating();
-
         // Block until the window gets an event
-        if !animating || self.suspended.get() {
+        if !self.animating() || self.suspended.get() {
             event_loop.set_control_flow(ControlFlow::Wait);
         } else {
             event_loop.set_control_flow(ControlFlow::Poll);
@@ -427,7 +426,7 @@ impl ApplicationHandler<WakerEvent> for App {
         );
         self.t = now;
 
-        if !matches!(self.state, AppState::Running(_)) {
+        if !matches!(self.state, AppState::Running(..)) {
             return;
         };
         let Some(window) = self.windows.values().next() else {
@@ -435,10 +434,8 @@ impl ApplicationHandler<WakerEvent> for App {
         };
         let window = window.clone();
 
-        let animating = self.is_animating();
-
         // Block until the window gets an event
-        if !animating || self.suspended.get() {
+        if !self.animating() || self.suspended.get() {
             event_loop.set_control_flow(ControlFlow::Wait);
         } else {
             event_loop.set_control_flow(ControlFlow::Poll);
diff --git a/ports/servoshell/desktop/events_loop.rs b/ports/servoshell/desktop/events_loop.rs
index ffe40eb8cb9..e47257d4f23 100644
--- a/ports/servoshell/desktop/events_loop.rs
+++ b/ports/servoshell/desktop/events_loop.rs
@@ -96,7 +96,7 @@ impl EventsLoop {
                     if !app.handle_events_with_headless() {
                         break;
                     }
-                    if !app.is_animating() {
+                    if !app.animating() {
                         *flag.lock().unwrap() = false;
                     }
                 }
diff --git a/ports/servoshell/desktop/headed_window.rs b/ports/servoshell/desktop/headed_window.rs
index f7986001c4d..d028d95cb73 100644
--- a/ports/servoshell/desktop/headed_window.rs
+++ b/ports/servoshell/desktop/headed_window.rs
@@ -14,7 +14,7 @@ use euclid::{Angle, Length, Point2D, Rotation3D, Scale, Size2D, UnknownUnit, Vec
 use keyboard_types::{Modifiers, ShortcutMatcher};
 use log::{debug, info};
 use raw_window_handle::{HasDisplayHandle, HasWindowHandle, RawWindowHandle};
-use servo::compositing::windowing::{AnimationState, WebRenderDebugOption, WindowMethods};
+use servo::compositing::windowing::{WebRenderDebugOption, WindowMethods};
 use servo::servo_config::pref;
 use servo::servo_geometry::DeviceIndependentPixel;
 use servo::webrender_api::ScrollLocation;
@@ -62,7 +62,6 @@ pub struct Window {
     /// A map of winit's key codes to key values that are interpreted from
     /// winit's ReceivedChar events.
     keys_down: RefCell<HashMap<LogicalKey, Key>>,
-    animation_state: Cell<AnimationState>,
     fullscreen: Cell<bool>,
     device_pixel_ratio_override: Option<f32>,
     xr_window_poses: RefCell<Vec<Rc<XRWindowPose>>>,
@@ -151,7 +150,6 @@ impl Window {
             webview_relative_mouse_point: Cell::new(Point2D::zero()),
             last_pressed: Cell::new(None),
             keys_down: RefCell::new(HashMap::new()),
-            animation_state: Cell::new(AnimationState::Idle),
             fullscreen: Cell::new(false),
             inner_size: Cell::new(inner_size),
             monitor,
@@ -570,10 +568,6 @@ impl WindowPortsMethods for Window {
         self.winit_window.set_cursor_visible(true);
     }
 
-    fn is_animating(&self) -> bool {
-        self.animation_state.get() == AnimationState::Animating
-    }
-
     fn id(&self) -> winit::window::WindowId {
         self.winit_window.id()
     }
@@ -772,10 +766,6 @@ impl WindowMethods for Window {
         self.device_pixel_ratio_override()
             .unwrap_or_else(|| self.device_hidpi_factor())
     }
-
-    fn set_animation_state(&self, state: AnimationState) {
-        self.animation_state.set(state);
-    }
 }
 
 fn winit_phase_to_touch_event_type(phase: TouchPhase) -> TouchEventType {
diff --git a/ports/servoshell/desktop/headless_window.rs b/ports/servoshell/desktop/headless_window.rs
index 76b846b3ebf..4ab202f1a56 100644
--- a/ports/servoshell/desktop/headless_window.rs
+++ b/ports/servoshell/desktop/headless_window.rs
@@ -9,7 +9,7 @@ use std::rc::Rc;
 
 use euclid::num::Zero;
 use euclid::{Length, Scale, Size2D};
-use servo::compositing::windowing::{AnimationState, WindowMethods};
+use servo::compositing::windowing::WindowMethods;
 use servo::servo_geometry::DeviceIndependentPixel;
 use servo::webrender_api::units::{DeviceIntSize, DevicePixel};
 use servo::{RenderingContext, ScreenGeometry, SoftwareRenderingContext};
@@ -20,7 +20,6 @@ use crate::desktop::window_trait::WindowPortsMethods;
 use crate::prefs::ServoShellPreferences;
 
 pub struct Window {
-    animation_state: Cell<AnimationState>,
     fullscreen: Cell<bool>,
     device_pixel_ratio_override: Option<Scale<f32, DeviceIndependentPixel, DevicePixel>>,
     inner_size: Cell<DeviceIntSize>,
@@ -50,7 +49,6 @@ impl Window {
             });
 
         let window = Window {
-            animation_state: Cell::new(AnimationState::Idle),
             fullscreen: Cell::new(false),
             device_pixel_ratio_override,
             inner_size: Cell::new(inner_size),
@@ -120,10 +118,6 @@ impl WindowPortsMethods for Window {
         self.fullscreen.get()
     }
 
-    fn is_animating(&self) -> bool {
-        self.animation_state.get() == AnimationState::Animating
-    }
-
     fn handle_winit_event(&self, _: Rc<RunningAppState>, _: winit::event::WindowEvent) {
         // Not expecting any winit events.
     }
@@ -157,8 +151,4 @@ impl WindowMethods for Window {
         self.device_pixel_ratio_override()
             .unwrap_or_else(|| self.device_hidpi_factor())
     }
-
-    fn set_animation_state(&self, state: AnimationState) {
-        self.animation_state.set(state);
-    }
 }
diff --git a/ports/servoshell/desktop/window_trait.rs b/ports/servoshell/desktop/window_trait.rs
index 62e31c73045..824d50f9c9c 100644
--- a/ports/servoshell/desktop/window_trait.rs
+++ b/ports/servoshell/desktop/window_trait.rs
@@ -28,7 +28,6 @@ pub trait WindowPortsMethods: WindowMethods {
     fn page_height(&self) -> f32;
     fn get_fullscreen(&self) -> bool;
     fn handle_winit_event(&self, state: Rc<RunningAppState>, event: winit::event::WindowEvent);
-    fn is_animating(&self) -> bool;
     fn set_title(&self, _title: &str) {}
     /// Request a new inner size for the window, not including external decorations.
     fn request_resize(&self, webview: &WebView, inner_size: DeviceIntSize)
diff --git a/ports/servoshell/egl/app_state.rs b/ports/servoshell/egl/app_state.rs
index ac747c8cf0e..faa91b4f23a 100644
--- a/ports/servoshell/egl/app_state.rs
+++ b/ports/servoshell/egl/app_state.rs
@@ -1,7 +1,7 @@
 /* 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 https://mozilla.org/MPL/2.0/. */
-use std::cell::{Ref, RefCell, RefMut};
+use std::cell::{Cell, Ref, RefCell, RefMut};
 use std::collections::HashMap;
 use std::rc::Rc;
 
@@ -11,7 +11,7 @@ use keyboard_types::{CompositionEvent, CompositionState};
 use log::{debug, error, info, warn};
 use raw_window_handle::{RawWindowHandle, WindowHandle};
 use servo::base::id::WebViewId;
-use servo::compositing::windowing::{AnimationState, EmbedderMethods, WindowMethods};
+use servo::compositing::windowing::{EmbedderMethods, WindowMethods};
 use servo::euclid::{Point2D, Rect, Scale, Size2D, Vector2D};
 use servo::servo_geometry::DeviceIndependentPixel;
 use servo::webrender_api::ScrollLocation;
@@ -86,9 +86,16 @@ struct RunningAppStateInner {
     focused_webview_id: Option<WebViewId>,
 
     context_menu_sender: Option<IpcSender<ContextMenuResult>>,
+
+    /// Whether or not the animation state has changed. This is used to trigger
+    /// host callbacks indicating that animation state has changed.
+    animating_state_changed: Rc<Cell<bool>>,
+}
+
+struct ServoShellServoDelegate {
+    animating_state_changed: Rc<Cell<bool>>,
 }
 
-struct ServoShellServoDelegate;
 impl ServoDelegate for ServoShellServoDelegate {
     fn notify_devtools_server_started(&self, _servo: &Servo, port: u16, _token: String) {
         info!("Devtools Server running on port {port}");
@@ -101,6 +108,10 @@ impl ServoDelegate for ServoShellServoDelegate {
     fn notify_error(&self, _servo: &Servo, error: ServoError) {
         error!("Saw Servo error: {error:?}!");
     }
+
+    fn notify_animating_changed(&self, _animating: bool) {
+        self.animating_state_changed.set(true);
+    }
 }
 
 impl WebViewDelegate for RunningAppState {
@@ -263,7 +274,10 @@ impl RunningAppState {
             .or_else(|| Url::parse("about:blank").ok())
             .unwrap();
 
-        servo.set_delegate(Rc::new(ServoShellServoDelegate));
+        let animating_state_changed = Rc::new(Cell::new(false));
+        servo.set_delegate(Rc::new(ServoShellServoDelegate {
+            animating_state_changed: animating_state_changed.clone(),
+        }));
 
         let app_state = Rc::new(Self {
             rendering_context,
@@ -276,6 +290,7 @@ impl RunningAppState {
                 webviews: Default::default(),
                 creation_order: vec![],
                 focused_webview_id: None,
+                animating_state_changed,
             }),
         });
 
@@ -345,6 +360,12 @@ impl RunningAppState {
         if !should_continue {
             self.callbacks.host_callbacks.on_shutdown_complete();
         }
+        if self.inner().animating_state_changed.get() {
+            self.inner().animating_state_changed.set(false);
+            self.callbacks
+                .host_callbacks
+                .on_animating_changed(self.servo.animating());
+        }
     }
 
     /// Load an URL.
@@ -684,10 +705,4 @@ impl WindowMethods for ServoWindowCallbacks {
     fn hidpi_factor(&self) -> Scale<f32, DeviceIndependentPixel, DevicePixel> {
         self.hidpi_factor
     }
-
-    fn set_animation_state(&self, state: AnimationState) {
-        debug!("WindowMethods::set_animation_state: {:?}", state);
-        self.host_callbacks
-            .on_animating_changed(state == AnimationState::Animating);
-    }
 }