run_webkit_tests.sh --per-tile-painting --platform=chromium compositing/culling/clear-fixed-iframe.html I haven't investigated what's going on but I suspect it's culling problem.
Fixed a culling bug here https://bugs.webkit.org/show_bug.cgi?id=81730 if there are rendersurfaces involved.
Well I've tracked it down so far to the SkPictureRecord class. saveLayer() isn't making something that gets exposed via the LayerIter. So the paint tracker has no idea that the current paint is being modified by a "transparencyLayer".
*** This bug has been marked as a duplicate of bug 82564 ***