Similar symptoms to bug 831335. Might be another regression from bug 827577. ###!!! ASSERTION: unexpected child list: 'Error', file ../../../layout/generic/nsBlockFrame.cpp, line 4733 ###!!! ASSERTION: How did that happen?: 'aFrameItems.IsEmpty()', file ../../../layout/base/nsCSSFrameConstructor.cpp, line 1309 ###!!! ASSERTION: Frames getting lost!: 'NS_SUCCEEDED(rv)', file ../../../layout/base/nsCSSFrameConstructor.cpp, line 1314 ###!!! ASSERTION: Dangling child list. Someone forgot to insert it?: '!FirstChild()', file ../../../layout/base/nsCSSFrameConstructor.cpp, line 652 Assertion failure: mPresArenaAllocCount == 0 (Some pres arena objects were not freed), at ../../../layout/base/nsPresShell.cpp:771

> Might be another regression from bug 827577. Seems very likely.

Rob, I think you should probably take this, I don't know this code at all.

Yeah, I don't know why I assigned this to you!

Robert, I don't quite understand this change. What causes the bug in this case?

In this testcase the fixed-pos container is the transformed overflow:auto element. The actual container frame is the scrollframe's child (the scrolled frame, an nsBlockFrame). That frame is *not* transformed, so ProcessFrameInsertions fails to put fixed-pos items on its abs-pos list.

Comment on attachment 710132 [details] [diff] [review] fix Ah, I see. r=me

Comment on attachment 710132 [details] [diff] [review] fix [Security approval request comment] How easily could an exploit be constructed based on the patch? Unknown. Do comments in the patch, the check-in comment, or tests included in the patch paint a bulls-eye on the security problem? Yes. Which older supported branches are affected by this flaw? Only trunk and Aurora are affected. If not all supported branches, which bug introduced the flaw? 827577 Do you have backports for the affected branches? If not, how different, hard to create, and risky will they be? This will be easy to put on Aurora. How likely is this patch to cause regressions; how much testing does it need? Dunno. It's a simple patch but this bit of the code has been regression-prone lately.

Comment on attachment 710132 [details] [diff] [review] fix sec-approval+

Could this be the underlying problem causing the crashes in bug 839263 / bug 837288 ?

https://hg.mozilla.org/integration/mozilla-inbound/rev/608f588cb3e9 (In reply to Mats Palmgren [:mats] from comment #11) > Could this be the underlying problem causing the crashes in bug 839263 / bug > 837288 ? I think so. We'll find out shortly :-).

Comment on attachment 710132 [details] [diff] [review] fix [Approval Request Comment] Bug caused by (feature/regressing bug #): 827577 User impact if declined: possible security-sensitive crashes Testing completed (on m-c, etc.): just landed Risk to taking this patch (and alternatives if risky): relatively low risk for regular browsing String or UUID changes made by this patch: none