Bug 317744 – [GTK][WPE] Skia Compositor: directly composited image is painted as a solid rectangle when partially offscreen

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Alejandro G. Castro]

With the Skia compositor an element whose contents are a directly composited image (e.g. a single transparent background image) is painted as an opaque rectangle instead of the image's real (partly transparent) shape when the element is partially clipped — for example positioned partly offscreen, so the composited layer's contents rect ends up with a negative origin.

Steps to reproduce:
  1. A <div> filled by a transparent PNG background, positioned with a negative top (so part of it is above the viewport), composited (e.g. will-change: opacity).         
  2. Observe the visible part with Skia composition enabled.

Result: the visible area is filled opaquely (a rectangle). Expected: the image's actual shape, with its transparent regions preserved. Rendering is correct with WEBKIT_USE_SKIA_FOR_COMPOSITION=0 (TextureMapper).

Apparently `SkiaCompositingLayer::paintContents()` the tiled-image SkShader's local matrix is built from the contents tile size and phase only, omitting the contents rect origin. With a non-zero (negative) origin the kRepeat shader samples the wrong part of the image.

[Alejandro G. Castro]

Pull request: https://github.com/WebKit/WebKit/pull/67770

[EWS]

Committed 315748@main (14dfefc6795a): <https://commits.webkit.org/315748@main>

Reviewed commits have been landed. Closing PR #67770 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/180522833>