[whatwg] Questions about ::backdrop

[Matt Falkenhagen]

I'm working on implementing the ::backdrop pseudo-element[1] for top
layer elements such as modal <dialog> and have some questions.

1. Should ::backdrop have the same properties as a real top layer
element, such as "Its containing block is the initial containing block."
and "Ancestor elements with overflow, opacity, masks, etc. cannot affect
it."?  It seems weird if a dialog is not affected by overflow, etc. and
its ::backdrop is. It also may be convenient to position and size the
backdrop relative to the ICB, like <dialog>. On the other hand, perhaps
it's convenient in some use cases for ::backdrop to be positioned and
sized relative to the <dialog>.

2. Clicking on a pseudo-element causes a click event to be dispatched to
its parent element. But this will make it unwieldy to implement
behaviors like dismissing a modal dialog or "bouncing" it for attention
when the area outside the dialog is clicked. Ideally you can easily
determine whether the dialog or its ::backdrop was clicked. Can we
perhaps use event.relatedTarget for this information?

[1] http://fullscreen.spec.whatwg.org/#::backdrop-pseudo-element