1. Set user_pref("full-screen-api.enabled", true); 2. Load the testcase 3. Click anywhere Result: ###!!! ASSERTION: Framesets should not be positioned and should not float: '!disp->IsAbsolutelyPositioned() && !disp->IsFloating()', file layout/generic/nsFrameSetFrame.cpp, line 1566

Hrm. So the issue is that we have some existing !important position/float rules in the UA stylesheet, but the full-screen rules are more specific... The assertion here dates back to bug 263406, but positioning of framesets was disallowed even before that.... We can probably remove this assertion for <frameset>, I suspect; I don't know of any obvious reasons why it can't be positioned/floated in the particular way that fullscreen stuff does it. But this raises an interesting question: what should happen for <frame>? Per spec, the insides of <frameset> don't participate in CSS layout at all, iirc; I'm pretty sure that trying to apply the fullscreen styles to <frame> won't work. Since I expect no one will ever do that, maybe the spec could just forbid fullscreening <frame>? And a similar question about <option>, actually.

Still happens on trunk after b99801e9a0c0 and friends modified the assertion condition: ###!!! ASSERTION: Framesets should not be positioned and should not float: '!disp->IsAbsolutelyPositionedStyle() && !disp->IsFloatingStyle()', file ../../../layout/generic/nsFrameSetFrame.cpp, line 1520

https://bugzilla.mozilla.org/show_bug.cgi?id=1472046 Move all DOM bugs that haven't been updated in more than 3 years and has no one currently assigned to P5. If you have questions, please contact :mdaly.