Auto merge of #9401 - glennw:raf-timing, r=jdm

Fixes additional calls to rAF.

Often, a rAF callback will request another rAF from the callback itself.

Previously, the constellation would quickly receive two messages saying
that there were no animations, and then there are animations again in the
situation above. This would make the compositor tick the new animation straight
away, causing strange fluctuations and timings in rAF callbacks.

Instead, only send the NoAnimationCallbacks message if the animation
callback queue is still empty after invoking the callbacks.

This fixes rAF timing, which now runs at the correct (vsync) framerate.

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1 files changed, 11 insertions, 5 deletions

diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index b6e1ce8bde0..b5672319837 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -1199,11 +1199,6 @@ impl Document {
         {
             let mut list = self.animation_frame_list.borrow_mut();
             animation_frame_list = Vec::from_iter(list.drain());
-
-            let ConstellationChan(ref chan) = self.window.constellation_chan();
-            let event = ConstellationMsg::ChangeRunningAnimationsState(self.window.pipeline(),
-                                                                       AnimationState::NoAnimationCallbacksPresent);
-            chan.send(event).unwrap();
         }
         let performance = self.window.Performance();
         let performance = performance.r();
@@ -1213,6 +1208,17 @@ impl Document {
             callback(*timing);
         }
 
+        // Only send the animation change state message after running any callbacks.
+        // This means that if the animation callback adds a new callback for
+        // the next frame (which is the common case), we won't send a NoAnimationCallbacksPresent
+        // message quickly followed by an AnimationCallbacksPresent message.
+        if self.animation_frame_list.borrow().is_empty() {
+            let ConstellationChan(ref chan) = self.window.constellation_chan();
+            let event = ConstellationMsg::ChangeRunningAnimationsState(self.window.pipeline(),
+                                                                       AnimationState::NoAnimationCallbacksPresent);
+            chan.send(event).unwrap();
+        }
+
         self.window.reflow(ReflowGoal::ForDisplay,
                            ReflowQueryType::NoQuery,
                            ReflowReason::RequestAnimationFrame);