[whatwg] Magical alignment and moving anchor targets

[Matt Falkenhagen]

The spec seems unclear on whether a magically aligned element[1] should
follow its anchor when its anchor moves, e.g., by dynamic style changes or
something like CSS animations.

I think we want the element to follow the anchor. If someone does something
like:

dialog.show(anchor)
anchor.style.top = ...

They'd probably expect dialog to end up where anchor is, rather than where
it was when show() was called.

Relatedly, it's not clear what happens if anchor is display: none or is not
in the document when show() is called, but later has a rendered box and is
in the document. And the reverse: if it is in the document when show() is
called and later is removed.

[1]
http://www.whatwg.org/specs/web-apps/current-work/multipage/commands.html#magically-aligned