Fixes the test case `/_mozilla/css/css-transition-cancel-event
.html`, which was failing under a specific circumstance.
The observed sequence of events during the failing test run looks like
this:
1. Transitions start in `div1` and `div2`.
2. `div1` generates a `transitionend` event.
3. The `transitionend` event handler removes `div2` from DOM, cancelling
its ongoing transition.
4. `div2` is supposed to generate a `transitioncancel` event in a timely
manner, which it does not. The test fails as a result.
What is going on here? Here's a possible explaination:
1. During one invocation of `ScriptThread::handle_msgs`...
2. In step 2, `ScriptThread::update_animations_send_events` -> `Document
::update_for_new_timeline_value` detects the completion of the
transition, and in response, pends the `transitionend` event.
3. In step 3, `ScriptThread::update_animations_send_events` ->
`Animations::send_pending_events` calls the `transitionend` handler.
4. The `transitionend` event handler removes `div2`, thereby cancelling
its ongoing transition and triggering a reflow.
5. Reflow takes place. During this, `Animations::do_post_reflow_update`
-> `Animations::handle_canceled_animations` pends the
`transitioncancel` event (precursor to step 4).
6. Having discovering that there was no running animation, `Animations::
do_post_reflow_update` calls `self.update_running_animation_presence
(_, false)`, which sends `AnimationState::NoAnimationsPresent`.
7. The invocation of `ScriptThread::handle_msgs` ends, and another
starts. It blocks waiting for events.
8. Meanwhile, the compositor receives `AnimationState::
NoAnimationsPresent` and stops further generation of animation ticks.
9. With no events to wake it up, the script thread is stuck waiting
despite having the pending `transitioncancel` event (step 4).
The HTML specification [says][1] that "an event loop must continually
run [...] as long as it exists" and does not say it can block if there
is nothing to do. Blocking is merely optimization in a user agent
implementation. Pending animation-related events must be processed every
time a "rendering opportunity" arises unless the user agent has a reason
to believe that it "would have no visible effect".
Skipping the processing of animation-related events would have visible
effect if such events are indeed present. The correct implementation in
Servo, therefore, would be to request more animation ticks so that such
events are processed in a subsequent tick.
[1]: https://html.spec.whatwg.org/multipage/#event-loop-processing-model
Stringify unknown JavaScript objects in global exception handlers
When turning DOM exceptions into `ErrorInfo` always try to stringify
the JavaScript value, even if it's an object that isn't a `DOMException`
or native exception. This means that exceptions that extend the `Error`
prototype are now stringified. The result is that test output for WPT
global assertion failures is more useful. For instance for the test
include-frames-from-child-same-origin-grandchild.sub.html:
Before:
```
uncaught exception: unknown (can't convert to string)
```
After:
```
uncaught exception: Error: assert_equals: expected 4 but got 3
```
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes.
When turning DOM exceptions into `ErrorInfo` always try to stringify
the JavaScript value, even if it's an object that isn't a `DOMException`
or native exception. This means that exceptions that extend the `Error`
prototype are now stringified. The result is that test output for WPT
global assertion failures is more useful. For instance for the test
include-frames-from-child-same-origin-grandchild.sub.html:
Before:
```
uncaught exception: unknown (can't convert to string)
```
After:
```
uncaught exception: Error: assert_equals: expected 4 but got 3
```
