[resize-observer] add possibility to observe offset changes

[HolgerJeromin]

In the entries of the ResizeObserver we have the contentRect and/or borderBoxSize.

For our application we need to call code when the offset and size of a Element has changed.
But sadly the ResizeObserver is not called when the position has changed.
For us it would be nice if I could add an option to observe the offset, too.

Observing the attributes with MutationObserver and checking a diff in all boundingRects has the downside of forced reflow in many cases.

ref #3550 where delivering offsets was rejected, as they are not observed.

[tabatkins]

So this sounds like it's a duplicate of #3550, then? Would you like to describe your use-case in that issue, and I'll go ahead and close this one?

[HolgerJeromin]

So this sounds like it's a duplicate of #3550, then?

Not in my view, see:

...to not include the offsets of the box that is being observed as the offsets are not actually be observed...

Adding these to the observe list was not discussed.

[mootari]

Shouldn't this issue be reopened, since it is not a duplicate of #3550 according to #3877 (comment)?