Bug 303401 – [scroll-anchoring] WPT test `scroll-animations/scroll-timelines/animation-with-root-scroller.html` fails with "Threaded Scroll-driven Animations" enabled

• Component: Scrolling
• Status: NEW

[Antoine Quint]

Once bug 303313 and bug 303317 both land, the WPT test `scroll-animations/scroll-timelines/animation-with-root-scroller.html` fails with "Threaded Scroll-driven Animations" enabled, but passes if "CSS Scroll Anchoring" is disabled.

This fails:

run-webkit-tests --debug --experimental-feature ThreadedScrollDrivenAnimationsEnabled=true imported/w3c/web-platform-tests/scroll-animations/scroll-timelines/animation-with-root-scroller.html

This passes:

run-webkit-tests --debug --experimental-feature ThreadedScrollDrivenAnimationsEnabled=true --experimental-feature CSSScrollAnchoringEnabled=false imported/w3c/web-platform-tests/scroll-animations/scroll-timelines/animation-with-root-scroller.html

The reason this reftest fails is that the whole document gets scrolled to the bottom of the page yielding the `RemoteProgressBasedTimeline` to have its current time set to 100% instead of the expected 50%. I traced the programmatic scroll being triggered under `Document::runScrollSteps()` and the call to `frameView->updateScrollAnchoringPositionForScrollableAreas()`.

[Radar WebKit Bug Importer]

<rdar://problem/165703614>

[Antoine Quint]

This also affects the following tests which are flakey ImageOnlyFailure on macOS… but not on iOS:

scroll-animations/css/scroll-timeline-default-quirks-mode.html [ ImageOnlyFailure Pass ]
scroll-animations/css/scroll-timeline-default-writing-mode-rl.html [ ImageOnlyFailure Pass ]
scroll-animations/css/scroll-timeline-default.html [ ImageOnlyFailure Pass ]
scroll-animations/css/scroll-timeline-frame-size-changed.html [ ImageOnlyFailure Pass ]
scroll-animations/css/scroll-timeline-inline-orientation.html [ ImageOnlyFailure Pass ]

The test scroll-animations/scroll-timelines/animation-with-root-scroller.html fails reliably on both macOS and iOS.

[Antoine Quint]

Further testing indicate that indeed this flakiness also applies to iOS, sorry for the wrong information earlier.

[Antoine Quint]

https://results.webkit.org/?suite=layout-tests&test=imported%2Fw3c%2Fweb-platform-tests%2Fscroll-animations%2Fscroll-timelines%2Fanimation-with-root-scroller.html shows this is no longer the case.

[Antoine Quint]

This progressed with 307007@main.

[Antoine Quint]

Pull request: https://github.com/WebKit/WebKit/pull/58616

[EWS]

Test gardening commit 307452@main (954eb7f71ef8): <https://commits.webkit.org/307452@main>

Reviewed commits have been landed. Closing PR #58616 and removing active labels.