Looks like WebGLRenderingContextBase needs this to be called, so that paintRenderingResultsToCanvas() knows that this is a 'display' and clears the back buffer.
Pull request: https://github.com/WebKit/WebKit/pull/4087
Committed 254259@main (0f38a84a5524): <https://commits.webkit.org/254259@main> Reviewed commits have been landed. Closing PR #4087 and removing active labels.
<rdar://problem/99678333>