fix(script): update animation timeline before processing pending events

This commit reverses the order of the `send_pending_events` and
`update_animation_timeline` calls in `ScriptThread::update_animations_
send_events` so that animation-related events pended by the latter are
processed by the former.

The new calling order is more compliant with the "update animations and
send events" algorithm steps from [the Web Animations specification][1].

The old implementation was prone to blocking for an indefinite period
while holding pending events. Due to complex interaction with other
events and timing behavior, I was only able to reproduce the problem
under the following conditions:

- *The `maybe_observe_paint_time` call in `LayoutThread::compute_abs_
  pos_and_build_display_list` is removed from the code*. While
  performance events may seem irrelevant to the issue, they would
  bombard the script thread with events. *Any* extra messages received
  would unblock the event loop and prevent the manifestation of the
  issue. (But, of course, we aren't supposed to count on that to avoid
  the issue.)

- Servo is running in a headless mode, which somehow makes it less
  likely for the script thread to receive a `TickAllAnimations` event
  after sending `AnimationState::NoAnimationsPresent`.

With the above conditions met and the stars aligned, you can reproduce
the problem by running the WPT test `/css/css-transitions/events-001.
html`.

[1]: https://drafts.csswg.org/web-animations-1/#update-animations-and-send-events

1 files changed, 2 insertions, 2 deletions

diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 506cd5d6bf2..a82edd61cc3 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1705,11 +1705,11 @@ impl ScriptThread {
     // Perform step 11.10 from https://html.spec.whatwg.org/multipage/#event-loops.
     fn update_animations_and_send_events(&self) {
         for (_, document) in self.documents.borrow().iter() {
-            document.animations().send_pending_events();
+            document.update_animation_timeline();
         }
 
         for (_, document) in self.documents.borrow().iter() {
-            document.update_animation_timeline();
+            document.animations().send_pending_events();
         }
     }