Bug 308165 – [macOS Debug] scrollingcoordinator/mac/latching/simple-page-rubberbands.html is a flaky text failure

• Component: Scrolling
• Status: NEW

[Diego De La Toba]

scrollingcoordinator/mac/latching/simple-page-rubberbands.html is a flaky text failure on MacOS Debug.


HISTORY:
https://results.webkit.org/?suite=layout-tests&test=scrollingcoordinator%2Fmac%2Flatching%2Fsimple-page-rubberbands.html


TEXT DIFF:
--- /Volumes/Data/worker/Apple-Tahoe-Debug-AppleSilicon-WK2-Tests/build/layout-test-results/scrollingcoordinator/mac/latching/simple-page-rubberbands-expected.txt
+++ /Volumes/Data/worker/Apple-Tahoe-Debug-AppleSilicon-WK2-Tests/build/layout-test-results/scrollingcoordinator/mac/latching/simple-page-rubberbands-actual.txt
@@ -6,12 +6,12 @@
 
 Swipe to the left
 PASS minXOffset < 0 is true
-PASS maxXOffset is 0
+FAIL maxXOffset should be 0. Was -4.
 PASS minYOffset is 0
 PASS maxYOffset is 0
 
 Swipe to the right
-PASS minXOffset is 0
+FAIL minXOffset should be 0. Was -3.
 PASS maxXOffset > 0 is true
 PASS minYOffset is 0
 PASS maxYOffset is 0


DIFF URL:
https://build.webkit.org/results/Apple-Tahoe-Debug-AppleSilicon-WK2-Tests/307769@main%20(911)/scrollingcoordinator/mac/latching/simple-page-rubberbands-pretty-diff.html


REPRODUCTION:
I was able to reproduce this failure with the following on macOS Tahoe Debug ToT:

run-webkit-tests --no-build --no-retry --no-show-results --exit-after-n-failures=1000 --expect-pass --iterations=1000 --force -f --clobber-old-results --debug scrollingcoordinator/mac/latching/simple-page-rubberbands.html

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

[Radar WebKit Bug Importer]

<rdar://problem/170669755>

[Diego De La Toba]

Pull request: https://github.com/WebKit/WebKit/pull/58954

[EWS]

Test gardening commit 307789@main (54d05e718e1b): <https://commits.webkit.org/307789@main>

Reviewed commits have been landed. Closing PR #58954 and removing active labels.

[Diego De La Toba]

Pull request: https://github.com/WebKit/WebKit/pull/58962

[Diego De La Toba]

I think the -1000 sentinel is problematic.

  With maxXOffset = -1000, for "Swipe to the left" with scroll events at -10, -8, -6, -4, -2:
  - maxXOffset = Math.max(-1000, -10, -8, -6, -4, -2) = -2
  - Check shouldBe('maxXOffset', '0') fails because -2 ≠ 0

  This only passes if a scroll event fires when rubber-band returns to position 0:
  - maxXOffset = Math.max(-1000, -10, ..., -2, 0) = 0

  But that final scroll event at position 0 doesn't always fire, causing flakiness.

  The fix: Initialize maxXOffset = 0 (and maxYOffset = 0).

  Caveat: If this is a real WebKit bug where scroll events SHOULD fire at position 0 per spec, this fix masks the issue since we no longer verify a scroll event fired at the final position.

  The test still verifies:
  - Rubber-band happened (scroll went negative)
  - Scroll didn't go in wrong direction (never went positive)