Created attachment 288471 [details] testcase See testcase, when hovering over the document, I get this assertion in current debug trunk build.
I also get this assertion if I Cmd+click a link to a bug and move my mouse over the new tab's content area before the page loads. It's pretty annoying.
http://mxr.mozilla.org/mozilla-central/source/layout/base/nsPresShell.cpp#5605 I guess we could try to find a target a bit harder - not only targetView = targetView->GetParent(), but also from parent presShell or something like that. This all would go away, I think, if we had only one view hierarchy (or no views at all). Anyway, that shouldn't be an assertion, but maybe a warning, if even that.
WFM on Mac using the testcase in comment 0. I can't reproduce the problem in comment 1 any more, either. Btw, bug 404798 turned this assertion into a warning. But I'm not even hitting the warning using the testcase or steps here.
We already have lots of crashtests like this, so in-testsuite+.