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Event retargeting algorithm doesn't work nicely if an event happens in older shadow root and an event listener would be invoked in a node in the younger shadow root. In current algorithm, it will retarget it's target to the shadow host. https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#event-retargeting
Let me rise this bug's priority. We have to resolve this issue to make the spec bug-free.
To resolve this issue nicely, I might have to use the result of the bug23770. https://www.w3.org/Bugs/Public/show_bug.cgi?id=23770
I think this can be addressed by re-defining tree of trees in https://github.com/w3c/webcomponents/commit/cdd1882c4739a997a93c99da43572db355d1793b.