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

@rniwa if a slot can be `ShadowRoot.activeElement`, what's the idiom to know the exact focused element?

```javascript
// some pseudo code included
focused = root.activeElement;
if (focused.tagName == 'SLOT') {
    for (node in focused.getAssignedNodes()) {
         if (node is really focused)
             return node;
    }
}
return focused;
```
(Probably there's a better way than mine :)

Or is it enough that a component knows *some element* under `<slot>` is focused, and it doesn't care which element it is?  If you have any good use case scenario, could you share?


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