Bug 309917 – [macOS iOS] imported/w3c/web-platform-tests/css/css-viewport/zoom/iframe-zoom-nested.html is a flaky image diff

• Component: New Bugs
• Status: NEW

[Diego De La Toba]

imported/w3c/web-platform-tests/css/css-viewport/zoom/iframe-zoom-nested.html is a flaky image diff.


HISTORY:
https://results.webkit.org/?suite=layout-tests&test=imported%2Fw3c%2Fweb-platform-tests%2Fcss%2Fcss-viewport%2Fzoom%2Fiframe-zoom-nested.html


IMAGE DIFF:
https://build.webkit.org/results/Apple-Tahoe-Release-AppleSilicon-WK2-Tests/309223%40main%20(2400)/imported/w3c/web-platform-tests/css/css-viewport/zoom/iframe-zoom-nested-diffs.html


IMAGES:
See attached.


REPRODUCTION:
Reproduced on macOS 26.2 Release ToT with the following:

run-webkit-tests --no-build --no-retry --no-show-results --exit-after-n-failures=1 --expect-pass --iterations=299 --force -f --release imported/w3c/web-platform-tests/css/css-viewport/zoom/iframe-zoom-nested.html

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

[Radar WebKit Bug Importer]

<rdar://problem/172508874>

[Diego De La Toba]

Created attachment 478673 [details]
images

[Diego De La Toba]

Test gardening pull request: https://github.com/WebKit/WebKit/pull/60591

[EWS]

Test gardening commit 309238@main (3bbb3a4f3657): <https://commits.webkit.org/309238@main>

Reviewed commits have been landed. Closing PR #60591 and removing active labels.

[Diego De La Toba]

Looks like listener registration should be moved to the top level so it's ready immediately.
Each nested leaf.html calls top.postMessage("leaf_loaded") on its own load event. These messages can arrive before the main window's load fires. If messages arrive before the listener exists, they're silently dropped and takeScreenshot() is never called, causing a timeout.

The reference files also have no waiting mechanism at all so subsequently the test harness periodically captures the screenshot
before the nested iframes have finished loading. This causes the flaky image failures.

[Diego De La Toba]

Pull request: https://github.com/WebKit/WebKit/pull/60595