Layout was using an outdated viewport rect that we set *after* doing
a layout due to resize. That meant that layout-in-response-to-resize
was always lagging behind the current size of the view.
The root of this problem was how Frame kept both a viewport rect
(with both scroll offset and size) and a frame size. To fix this,
only store the viewport scroll offset, and always use the frame size.
This way they can't get out of sync and the problem goes away. :^)
Fixes#4250.
15e35b0d71