[iOS Sim] Layout Test scrollingcoordinator/ios/ui-scroll-fixed.html is a flaky failure

• Component: Tools / Tests
• Status: RESOLVED FIXED

[Shawn Roberts]

The following layout test is flaky on iOS Simulator Release WK2

scrollingcoordinator/ios/ui-scroll-fixed.html

Probable cause:

Test was added in https://trac.webkit.org/changeset/242683/webkit 

A note was made on previous bug, but a fix does not appear to have been made.

Flakiness Dashboard:

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=scrollingcoordinator%2Fios%2Fui-scroll-fixed.html

Diff:

https://build.webkit.org/results/Apple%20iOS%2012%20Simulator%20Release%20WK2%20(Tests)/r243869%20(3458)/scrollingcoordinator/ios/ui-scroll-fixed-diff.png

[Radar WebKit Bug Importer]

<rdar://problem/49612867>

[Shawn Roberts]

Marked flaky in https://trac.webkit.org/changeset/243881/webkit while waiting for a fix

[Simon Fraser (smfr)]

I can reproduce. This only seems to happen if amount scrolled (50px) is identical to the top of #container, so we must have an early return in code somewhere.

[Simon Fraser (smfr)]

Created attachment 381146 [details]
Patch

[Antti Koivisto]

Comment on attachment 381146 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=381146&action=review

> LayoutTests/ChangeLog:14
> +        This test hit a really obscure bug where a combination of an immediate scroll, and
> +        an ancestor reposition left the layer position of a position:fixed layer unchanged.
> +        The position of this layer in the UI process had been previously modified by
> +        the scrolling tree for the scroll, but because the WebContent-side mutations left
> +        the actual position unchanged, we'd never apply a new position via a commit, so left
> +        the layer in the wrong location.

Would this be fixed by a web->ui message that that signals we did a compositing layer update but nothing changed, so we are skipping the commit? This would just call applyLayerPositionsAfterCommit() to check for m_wasScrolledByDelegatedScrollingSincePreviousCommit bit.

> LayoutTests/ChangeLog:19
> +        Since this is so hard to hit with noisy user scrolling, just change the test to avoid
> +        the perfect storm of scrolls and offsets.

Since we have managed to make a test for this it bit sad to let it go waste.

[Simon Fraser (smfr)]

I filed bug 203112 to cover the underlying bug.

[WebKit Commit Bot]

The commit-queue encountered the following flaky tests while processing attachment 381146 [details]:

imported/w3c/web-platform-tests/mathml/presentation-markup/operators/mo-form-fallback.html bug 203076 (authors: [e-mail] and [e-mail])
imported/w3c/web-platform-tests/websockets/bufferedAmount-unchanged-by-sync-xhr.any.worker.html bug 202003 (author: [e-mail])
The commit-queue is continuing to process your patch.

[WebKit Commit Bot]

Comment on attachment 381146 [details]
Patch

Clearing flags on attachment: 381146

Committed r251251: <https://trac.webkit.org/changeset/251251>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.