Previously, when the mouse was released after dragging a scrollbar,
its highlight was not dropped (if the mouse cursor was still inside
the viewport). This seems to be because the currently hovered control
only gets updated when the mouse is moved.
This commit fixes the dropping of the cosmetic highlight by running
the check for whether the currently hovered control has changed on
mouse-clicks, in addition to to the existing mouse-movements.
e8804b9978