Created attachment 258561 [details] testcase The testcase gives me this testcase on current debug trunk build: ###!!! ASSERTION: no common ancestor at all???: 'parent', file c:/mozilla/mozill a/layout/base/nsLayoutUtils.cpp, line 349
Does that happen only if the display is set on the body tag?
No, it happens when the parent of the iframe is display: table-caption, apparently.
I can reproduce with today's minefield on linux.
WFM on mozilla-central on Mac. Martijn or bc, are you still seeing this?