Created attachment 396443 [details] Reduction Element backgrounds fail to paint sometimes when scrolling in gmail with async overflow scrolling enabled.
rdar://problem/60523869
I think this is about painting with stale clip rects (which are probably computed using some stale main-thread scrolling offset).
We paint out-of-view tiles but RenderBlock::paint() short-circuits because they are clipped out (note the iOS #ifdef), then we later scroll those tiles back into view and they are missing content.
RenderBox::overflowRectForPaintRejection() is supposed to handle this, but m_overflow is null for some reason.
Created attachment 396478 [details] Patch
Created attachment 396494 [details] Patch
https://trac.webkit.org/changeset/260118/webkit