[Webkit-unassigned] [Bug 249684] New: Prevent the WindowEventLoop tasks from running between trigger an rendering update and the end of that update
bugzilla-daemon at webkit.org
bugzilla-daemon at webkit.org
Tue Dec 20 16:26:11 PST 2022
https://bugs.webkit.org/show_bug.cgi?id=249684
Bug ID: 249684
Summary: Prevent the WindowEventLoop tasks from running between
trigger an rendering update and the end of that update
Product: WebKit
Version: WebKit Nightly Build
Hardware: Unspecified
OS: Unspecified
Status: NEW
Severity: Normal
Priority: P2
Component: DOM
Assignee: webkit-unassigned at lists.webkit.org
Reporter: simon.fraser at apple.com
I was trying to implement `testRunner.renderingUpdate()` so you could write a test like:
window.addEventListener('load', async () => {
await testRunner.renderingUpdate();
debug('rendering update complete');
// do stuff
}, false);
However, that turned out have flakey behavior with respect to the ordering of rAF callbacks:
window.addEventListener('load', async () => {
requestAnimationFrame(() => {
debug('requestAnimationFrame callback');
requestAnimationFrame(() => {
debug('second requestAnimationFrame callback, which should always come after the "rendering update complete"');
});
});
await testRunner.renderingUpdate();
debug('rendering update complete');
}, false);
Sometimes the second rAF callback happened before "rendering update complete" and sometimes after. This depended on the timing of the microtask callbacks fired from WindowEventLoop, which I believe handles the resume of the load function when the `renderingUpdate` Promise is resolved.
The source of unpredictability which I identified was the timing of the zero-delay timer in WindowEventLoop. The incorrect behavior happened when the WindowEventLoop timer fired between `TiledCoreAnimationDrawingArea::scheduleRenderingUpdateRunLoopObserver()` and `TiledCoreAnimationDrawingArea::updateRenderingRunLoopCallback()`. There's already code that tries to avoid timing firing delaying the runloop observer for long periods of time (`breakToAllowRenderingUpdate()`), so we can piggyback off that to avoid this timer firing between the scheduling and the fire of the CFRunLoopObserver that controls the rendering update.
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