UI Events - Mouse Events and iframe targetting

It seems that processing mouse [move|up] events have 3 different
implementations when involving an iframe in each rendering engine.

I'm soliciting comments and hope we can work on defining some behavior and
converging our implementations to benefit the web author.

Specifically when an mouse down event occurs inside an iframe; the
subsequent mouse moves/ups are targeted at different frames when the mouse
moves outside the bounds of the iframe.

1) Chrome targets the frame that generated the mouse down frame sometimes
(but has side effects with prevent default; see http://crbug.com/269917; we
need to fix this).
2) Firefox targets the mouse down frame unless you move into a sibling
iframe
3) IE 11 & Edge target the topmost frame under the mouse move regardless of
the mouse down operation.

We need to get some clarity written into the specification. As the spec
indicates the target is:


   - Event.target
   <http://www.w3.org/TR/DOM-Level-3-Events/#widl-Event-target>: topmost
   event target
   <http://www.w3.org/TR/DOM-Level-3-Events/#glossary-topmost-event-target>


Likewise in Edge/IE 11 you can have a mouse down event in the iframe; but
the mouse up event occurs in the main frame.

All browsers do not generate a mouse leave/enter on the iframe document
when the mouse leaves or enters the iframe when a mouse button is
depressed; this can leave the iframe in an inconsistent state possibly.

So I would expect that the IE 11/Edge implementation is spec compliant.
However this doesn't necessarily seem correct to the web author point of
view.

I can see why the target is defined is set to be the iframe that handled
the mouse down event because if you are dragging something around you might
not want to start processing mouse move events on the outer frame until the
mouse is released.  The crbug references google maps embedded inside a page
as a use case that occurs in the field today.

With respect to hover processing:
1) Chrome doesn't generate hover events for items outside of the iframe
during a mouse down. (Seems incorrect).
2) Firefox does the interesting behaviour of generating hover events for
items not in the parent frame but in sibling frames.
3) IE 11 & Edge generate hover on whatever is under the mouse.

The IE 11/Edge implementation seems straight forward. Perhaps some folks
can confirm that it is as I think it to be.

The Firefox implementation mystifies me why it behaves different between
the main frame and a sibling iframe is interesting. Perhaps I'm flawed in
my reproduction step; but some explanation behind this would be appreciated.

The Chrome/WebKit implementation makes sense a little bit of sense to me as
well; granted it has some weird bugs like hover and prevent default.

dave.