* scrollLeft/scrollTop returned values of parent or even document root
Only the scroll of the node itself is returned. Otherwise 0.0.
* Scrolling via script had set viewport.
This resulted in other nodes appearing scrolled.
Now scroll_offsets are updated with correct node id.
These bugs caused other odd behavior like both body and
document.documentElement being scrolled or the view for scrolled
elements jumping.
see #10947
Node::scroll_area should check if the element is potentially scrollable
Three tests use `document.scrollingElement` which was not supported at
the time of writing. They are, hence, expected to FAIL.
The last expected FAIL should be fixed by #10947.
Edited some comments, message texts.
