Created attachment 352107 [details]
Attached test case, when loaded on iOS, shows bad problems where the position of the logging box jumps around, and the content doesn't get fully tilted, and it can't be scrolled.
Even with frame flattening off, I see two issues with this test content on iPad:
1. On first load, the position:fixed in the iframe is missing
2. After scrolling the iframe for a bit, we lose all the tiles in the logging element.
My guess is that GraphicsLayer geometry isn't getting updated somehow, triggering incorrect visible rects. Maybe this is related to boundsOrigin vs. position differences.
@smfr: Not sure how it relates to bug 194433, but AFAIK my remaining tests for position: fixed/sticky when async frame scrolling is enabled still don't pass.
https://trac.webkit.org/changeset/243539/webkit fixes position:fixed inside the iframe. I still see missing tiles in the scroller sometimes.
Appears to be fixed.