In bug 1116588 I added the check at  as a way of getting event-regions on opacity:0 items but without too much of a performance hit in cases where we didn't need it. However tn pointed out recently that children elements might set pointer-events back to auto or something, and right now the code will just skip the entire subtree, missing those. For correctness we should handle that case but practically speaking it's a pretty edge case that I'm not sure we'll run into anytime soon. Filing this bug so we don't forget about it.  http://mxr.mozilla.org/mozilla-central/source/layout/generic/nsFrame.cpp?rev=1a1b8b9ec10a#1940
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