Bug 308667 – [iOS] webanimations/threaded-animations/scroll-timeline-root-scroller.html is a flaky text failure

• Status: NEW
• Component: New Bugs

[Diego De La Toba]

webanimations/threaded-animations/scroll-timeline-root-scroller.html is a flaky text failure on both release and debug however, release flakes do not show on bots and are extremely flaky.


HISTORY:
https://results.webkit.org/?suite=layout-tests&test=webanimations%2Fthreaded-animations%2Fscroll-timeline-root-scroller.html&platform=ios&recent=false&style=debug&recent=false


DIFF:
https://build.webkit.org/results/Apple-iOS-26-Simulator-Debug-WK2-Tests/308173%40main%20(779)/webanimations/threaded-animations/scroll-timeline-root-scroller-pretty-diff.html


DIFF URL:
--- /Volumes/Data/worker/Apple-iOS-26-Simulator-Debug-WK2-Tests/build/layout-test-results/webanimations/threaded-animations/scroll-timeline-root-scroller-expected.txt
+++ /Volumes/Data/worker/Apple-iOS-26-Simulator-Debug-WK2-Tests/build/layout-test-results/webanimations/threaded-animations/scroll-timeline-root-scroller-actual.txt
@@ -1,3 +1,3 @@
 
-PASS The timeline current time updates as its source is scrolled with a root scroller
+FAIL The timeline current time updates as its source is scrolled with a root scroller assert_equals: After scrolling to 700, the timeline time is 50.00% expected "50.00%" but got "0.00%"
 

REPRODUCTION: 
I was able to reproduce this on iOS 26.2 debug and release ToT with the following:

Release: 
run-webkit-tests --no-build --no-retry --no-show-results --exit-after-n-failures=1 --expect-pass --iterations=2000 --force -f --clobber-old-results --ios-simulator --release webanimations/threaded-animations/scroll-timeline-root-scroller.html --udid < 3 udids >

Debug: 
run-webkit-tests --no-build --no-retry --no-show-results --exit-after-n-failures=1 --expect-pass --iterations=1000 --force --clobber-old-results --ios-simulator --debug webanimations/threaded-animations/scroll-timeline-root-scroller.html

I am going to mark expectations as pass fail while this pends investigation.

[Radar WebKit Bug Importer]

<rdar://problem/171197721>

[Diego De La Toba]

Pull request: https://github.com/WebKit/WebKit/pull/59446

[EWS]

Test gardening commit 308231@main (0733cc3c8b6c): <https://commits.webkit.org/308231@main>

Reviewed commits have been landed. Closing PR #59446 and removing active labels.

[Diego De La Toba]

This could be a test issue where one UIHelper.renderingUpdate() is not sufficient since it waits for a local animation frame + setTimeout in the web process but may not guarantee the UI process has handled the scroll thus resulting in the test not receiving a scroll. This is also supported by the fact that this is much flakier when running at high cpu usage and debug. 

Adding another UIHelper.renderingUpdate() allows the test to pass all iterations - even in parallel so I am going to put this fix up as a pr. But I do think it's worth mentioning this could be masking an actual WebKit issue where timeline updates are not happening when they should.

[Diego De La Toba]

Pull request: https://github.com/WebKit/WebKit/pull/59461