Bug 313721 – UseSkiaForComposition: compositing/update-compositing-rects-with-sharing-descendants.html fails

• Status: RESOLVED
• Resolution: FIXED
• Component: WebKitGTK

[Fujii Hironori]

If UseSkiaForComposition is enabled, compositing/update-compositing-rects-with-sharing-descendants.html fails.

[Fujii Hironori]

Pull request: https://github.com/WebKit/WebKit/pull/63956

[Fujii Hironori]

Created attachment 479387 [details]
test case

[Fujii Hironori]

Created attachment 479407 [details]
test 2

[Fujii Hironori]

Created attachment 479414 [details]
test 3

compositing/update-compositing-rects-with-sharing-descendants.html fails because the blue box isn't painted.
However, with MiniBrowser, the blue box is painted.
The difference between MiniBrowser and WebKitTestRunner is AsyncOverflowScrolling. By disabling AsyncOverflowScrolling, the artifact is reproducible with MiniBrowser.

> ./Tools/Scripts/run-minibrowser --gtk --release --features=-AsyncOverflowScrolling ./LayoutTests/compositing/update-compositing-rects-with-sharing-descendants.html

If AsyncOverflowScrolling is enabled, overflow:scroll div also gets a composited layer.
And, the parent isolation:isolate layer and the child overflow:scroll layer overlaps.

If overlap region is larger than non-overlap region, the non-overlap region is merged into the overlap region.
https://github.com/WebKit/WebKit/blob/e3d5d9821fa8886c33382d72f35d0d201f24536f/Source/WebCore/platform/graphics/skia/SkiaCompositingLayer.cpp#L920-L925

This is the reason MiniBrowser paints the blue box.

If I enlarge the blue box, MiniBrowser also doesn't paint the blue box.

[EWS]

Committed 312511@main (96de099fd31a): <https://commits.webkit.org/312511@main>

Reviewed commits have been landed. Closing PR #63956 and removing active labels.