Re: [w3c/webcomponents] Discussion: child elements cannot determine whether they are "active" or not (#817)

I'd be concerned with exposing such callbacks because different engines have different kinds of behavior here. For example Gecko can end up rebuilding ancestor layout trees, if it turns out that dynamically inserting something is hard, like with some IB-split cases and table cases that WebKit and Blink get wrong. A naive way of defining this would result in a "detached" and "attached" callback for no reason.

A callback for getting into the flat tree and out, or such, seems more sensible to me, or alternatively, maybe, an explicit API that gives you whether you're rendered? There's other APIs that already expose that (`getClientRects()` returns an empty list if you don't have a layout box for example).

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