<rdar://123831236> Backdrop-filter makes the nearest suitable ancestor a backdrop-root, which forces compositing. Most commonly, this is the document element, and we add an extra full-screen compositing layer. This is done to ensure that the compositing layer that the backdrop is read from doesn't contain the background colour the view behind the document. If the document element has an opaque background, then it will occlude the view's background color, and we can skip creating a compositing layer.
Pull request: https://github.com/WebKit/WebKit/pull/26211
Committed 276749@main (7ffc7f9fd02a): <https://commits.webkit.org/276749@main> Reviewed commits have been landed. Closing PR #26211 and removing active labels.