CSS3 3D Transform's "transform-style" property, when used with 'flag' value has recently broken on WebKit. You can verify by running the example from Surfin' Safari blog [1] and noticing the weird white boxes drawn while hovering mouse (or a single tap on mobile mode, as confirmed by Rafael) over the rotating div. I'm going to bisect the code to find the dangling patch. Links: [1] https://www.webkit.org/blog-files/3d-transforms/transform-style.html
I've bisected the code and found out that a fix for a patch that avoids redundant state clipping was causing the issue ( http://trac.webkit.org/changeset/144290 ), from bug 111065. Reverting the patch solves the issue, however like Allan commented, it breaks clipping on some Apple HTML5 examples (i.e. http://www.apple.com/html5/showcase/transitions/ ).
Created attachment 191834 [details] Patch Proposed patch
Comment on attachment 191834 [details] Patch View in context: https://bugs.webkit.org/attachment.cgi?id=191834&action=review > Source/WebCore/platform/graphics/texmap/TextureMapperGL.cpp:1097 > + m_clipStack.apply(m_context3D.get(), false); This is a boolean trap. I prefer to have apply(...) and applyIfNeeded(...). From bindDefaultSurface we'd call apply, and from everywhere else applyIfNeeded.
Created attachment 191976 [details] Patch Added applyIfNeeded() to avoid boolean trap as reviewed by Noam
Created attachment 191980 [details] Patch ChangeLog typo fixes as reviewed by Allan
Comment on attachment 191980 [details] Patch LGTM
Created attachment 191981 [details] Patch Patch for landing (last minute ChangeLog typo fix)
Comment on attachment 191981 [details] Patch Clearing flags on attachment: 191981 Committed r145071: <http://trac.webkit.org/changeset/145071>
All reviewed patches have been landed. Closing bug.