In bug 1175631 comment 5 smaug suggested adding a MOZ_ASSERT(mFrameElement) to TabParent::HandleEvent. I did so but it caused test failures  when it landed on inbound. I ended up backing out the assertion, but presumably this is not expected and needs investigation.  https://treeherder.mozilla.org/logviewer.html#?job_id=10930462&repo=mozilla-inbound
Yes, because that hints there are leaks - not necessarily, but likely.
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