[webkit-reviews] review requested: [Bug 5909] overlapping element leaves trail when scrolling iframe (affects Google Docs) : [Attachment 29762] Test for overlapping layers during painting and enable/disable blitting accordingly

[bugzilla-daemon at webkit.org]

mitz at webkit.org has asked  for review:
Bug 5909: overlapping element leaves trail when scrolling iframe (affects
Google Docs)
https://bugs.webkit.org/show_bug.cgi?id=5909

Attachment 29762: Test for overlapping layers during painting and
enable/disable blitting accordingly
https://bugs.webkit.org/attachment.cgi?id=29762&action=review

------- Additional Comments from mitz at webkit.org
This patch lets a widgets (or in fact, any object, so in the future perhaps
also overflow areas and media elements) make overlap-testing requests a paint
time. The result is delivered by the end of the paint operation. There is no
separate walk of the layer tree, so the results are guaranteed to reflect what
was actually painted.

Transforms and reflections are not handled, but that is not a regression of
course.

In the future, the interface can change so that setOverlapTestResult() takes
two rects: the rect from the request and the overlapping layer's rect. This
will allow the requester to cache its overlapped region and update it. Frames
could use this to enable partial blitting, for example.