Re: [webcomponents] [Shadow] activeElement behavior seems to break encapsulation (#358)

I think what Hayato said - that only focusable elements can be assigned to a Document / ShadowRoot's `activeElement` - makes sense / was the original rationale for not focusing slots.

This is probably outside the scope of this particular bug but it might still be worth considering if there should be a simple(r) way to determine more information about if / where a focused element deeper in the composed tree exists from within a ShadowRoot (without giving away information external to it). To my understanding, the event propagation path makes it possible to capture focus / blur events anywhere deeper in the composed tree than the node you're listening on (even if those events' targets are reprojected from somewhere shallower in the tree-of-trees). Given that, it's kind of odd that this information is first made available within the ShadowRoot but then lost after the event has stopped propagating. But, maybe it *should* be lost for the same reason as this change to activeElement?

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https://github.com/w3c/webcomponents/issues/358#issuecomment-179469089