Bug 309019 – REGRESSION(308215@main): [iOS] imported/w3c/web-platform-tests/dom/events/scrolling/scrollend-event-fired-for-programmatic-scroll.html?include=root-scrollBy-auto is a flaky text failure

• Component: Scrolling
• Status: NEW

[Diego De La Toba]

imported/w3c/web-platform-tests/dom/events/scrolling/scrollend-event-fired-for-programmatic-scroll.html?include=root-scrollBy-auto is a flaky text failure on iOS


HISTORY:
https://results.webkit.org/?suite=layout-tests&test=imported%2Fw3c%2Fweb-platform-tests%2Fdom%2Fevents%2Fscrolling%2Fscrollend-event-fired-for-programmatic-scroll.html%3Finclude%3Droot-scrollBy-auto


DIFF:
--- /Volumes/Data/worker/Apple-iOS-26-Simulator-Release-WK2-Tests/build/layout-test-results/imported/w3c/web-platform-tests/dom/events/scrolling/scrollend-event-fired-for-programmatic-scroll_include=root-scrollBy-auto-expected.txt
+++ /Volumes/Data/worker/Apple-iOS-26-Simulator-Release-WK2-Tests/build/layout-test-results/imported/w3c/web-platform-tests/dom/events/scrolling/scrollend-event-fired-for-programmatic-scroll_include=root-scrollBy-auto-actual.txt
@@ -1,3 +1,3 @@
 
-PASS Tests scrollend event for calling scrollBy with behavior 'auto' on root.
+FAIL Tests scrollend event for calling scrollBy with behavior 'auto' on root. assert_equals: HTML.scrollBy scrollLeft expected 200 but got 0
 

DIFF URL:
https://build.webkit.org/results/Apple-iOS-26-Simulator-Release-WK2-Tests/308467%40main%20(1971)/imported/w3c/web-platform-tests/dom/events/scrolling/scrollend-event-fired-for-programmatic-scroll_include=root-scrollBy-auto-pretty-diff.html


REPRODUCTION:
I was able to reproduce on iOS 26.2 with the following:

run-webkit-tests --no-build --no-retry --no-show-results --exit-after-n-failures=1 --expect-pass --iterations=1000 --force --ios-simulator --release imported/w3c/web-platform-tests/dom/events/scrolling/scrollend-event-fired-for-programmatic-scroll.html

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

[Radar WebKit Bug Importer]

<rdar://problem/171569422>

[Diego De La Toba]

Pull request: https://github.com/WebKit/WebKit/pull/59750

[EWS]

Test gardening commit 308493@main (81731012a37e): <https://commits.webkit.org/308493@main>

Reviewed commits have been landed. Closing PR #59750 and removing active labels.

[Simon Fraser (smfr)]

This happens because we get into WebPage::updateVisibleContentRects() when ViewUpdateDispatcher::visibleContentRectUpdate() fires, which is unpredictable, and that does not go through the logic in RemoteScrollingCoordinator::scrollUpdateForNode() that suppresses user scrolls while a programmatic scroll response is pending.

The base issue here is that we lose the user/programmatic distinction for root scroller changes coming in via VisibleContentRects on iOS.

[Simon Fraser (smfr)]

Even if we make ViewUpdateDispatcher more deterministic (pulling update rects at the start of each rendering update), things are still non-deterministic because of how the UI process batches `-[WKWebView _updateVisibleContentRects]` on a per CoreAnimation commit basis.