[webcomponents] Should relatedTargetScoped flag be set on mousedown/mouseup events? (#433)

Do we really want `mousedown` and `mouseup` events to have `relatedTargetScoped` flag set?  The following [definition in the current spec](http://w3c.github.io/webcomponents/spec/shadow/#dfnReturnLink-0) seems to indicate any event that uses [`MouseEvent` interface](https://w3c.github.io/uievents/#mouseevent) to have this flag set:

> A trusted event's relatedTargetScoped flag must be initialized to true if the event has a relatedTarget property

Is this an intentional (that any mousedown/mouseup would not propagate out of a shadow root)?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/433