Bug 307949 – Rotating bounding box invalidation does not redraw a border on other element

• Status: RESOLVED
• Resolution: FIXED
• Component: Layout and Rendering

[Jonathan Deutsch]

Created attachment 478358 [details]
Reproduction HTML Document

A div that has rotates can cause a region to be invalidated for repainting. However, if there's another element underneath that has a border, this border may not be repainted.

Please see the simple repro case html document and video (Safari vs Chrome).

The issue can be reproduced on:
	macOS 15.7.4 (24G517) and Safari Version 26.3 (20623.2.7.18.1)
	macOS 15.7.4 (24G517) and Safari Technology Preview Release 237 (WebKit 20625.1.3.19.2)
	macOS 26.1 Beta (25B5042k) and Safari Version 26.1 (21622.2.5)
	
The issue does not reproduce on:
	macOS 14.4 (23E214) and Safari Version 17.4 (19618.1.15.11.12)
	Chrome
	Firefox
	WKWebView inside of a native mac app on macOS 15.7.4

This bug affects users of the Tumult Hype application for macOS:
https://tumult.com/hype/

[Jonathan Deutsch]

Created attachment 478359 [details]
Screenshot of the bug in Safari

[Jonathan Deutsch]

Created attachment 478360 [details]
Video showing Chrome (left) vs Safari with bug (right)

[Jonathan Deutsch]

Note that adding `will-change: transform;` to the #small-box CSS will workaround the issue.

[Radar WebKit Bug Importer]

<rdar://problem/170438404>

[Ahmad Saleem]

I am able to reproduce on WebKit ToT (Minibrowser - 307621@main).

[Ahmad Saleem]

*** Chrome Canary 147 (147.0.7688.0 (Official Build) canary (arm64) ) ***

Does not reproduce.

*** Fireofx Nightly 149 (20260215090851) ***

Does not reproduce.

[Simon Fraser (smfr)]

This is a repaint bug.

[Nipun Shukla]

Pull request: https://github.com/WebKit/WebKit/pull/59049

[EWS]

Committed 307930@main (54b6555c4fa6): <https://commits.webkit.org/307930@main>

Reviewed commits have been landed. Closing PR #59049 and removing active labels.