Bug 309097 – Changing color-scheme doesn't repaint background of composited iframes

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Kiet Ho]

Created attachment 478552 [details]
Test case

See test case.

Steps to reproduce:
* Open the test case
* Click "toggle color-scheme" to switch color-scheme of the main page from light to dark
* Observe that the iframe's background is black instead of white.

CSS Color Adjustment Module Level 1 [1] dictates that if the used color scheme of the child frame's owner element is different than the used color scheme of the child frame, the background of the child frame must be opaque and not transparent [2]:

> In order to preserve expected color contrasts, in the case of embedded documents typically rendered over a transparent canvas (such as provided via an HTML iframe element), if the used color scheme of the element and the used color scheme of the embedded document’s root element do not match, then the UA must use an opaque canvas of the Canvas color appropriate to the embedded document’s used color scheme instead of a transparent canvas. 

WebKit implemented this in 285606@main [3], but it's a bit incomplete. If color-scheme of the owner element changes, and the child iframe is composited (e.g because of scrolling), then its background doesn't get repainted. It repaints fine if the frame is non-composited.

Fixing this might also fix css/css-color-adjust/rendering/dark-color-scheme/color-scheme-iframe-dynamic.html in WPT.

[1]: https://drafts.csswg.org/css-color-adjust/
[2]: https://drafts.csswg.org/css-color-adjust/#:~:text=In%20order%20to%20preserve%20expected%20color%20contrasts
[3]: https://github.com/WebKit/WebKit/commit/fe5bf113532e9e7753f2ba2c0856bdd097c82f96

[Radar WebKit Bug Importer]

<rdar://problem/171658244>

[Kiet Ho]

[3] should be 241325@main

[Kiet Ho]

Pull request: https://github.com/WebKit/WebKit/pull/60237

[Kiet Ho]

Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/58557

[EWS]

Committed 309567@main (bf18b1f243b2): <https://commits.webkit.org/309567@main>

Reviewed commits have been landed. Closing PR #60237 and removing active labels.