PointerEvents and iframes

A classic problem with using iframes is that mouse events will not fire
over them. This is particularly troublesome for user interaction and
dragging motions.

I've had a few conversations about trying to let iframes use event
retargeting instead, similar to how events bubble in Shadow DOM (
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#event-retargeting),
and have been told that there is too much legacy around the "iframe
blackhole" to change for mouse events.

However, since Pointer Events are brand new, maybe we could add this iframe
event retargeting to the spec. In this scenario, a Pointer Event that
bubbles through an iframe will have the target changed to the iframe,
negating the possibility of an information leak to the parent window.

Thoughts?