This fixes a bug where users of the scrollbar had to be very careful
not to move the mouse outside the viewport, otherwise the scrollbar
would drop its drag-action and stop scrolling until clicked again.
The existing behaviour had the side-effect of also dropping the
cosmetic highlighting of the scrollbar (in addition to the dragging),
for the specific case where the mouse was move outside the window.
The previous behaviour did nothing to remove the highlight if the
mouse was released (but not moved) inside the viewport.
This separate issue with the lingering highlight of the scrollbar
(until a mouse-movement action is performed inside the viewport) is
fixed in an immediate followup to this commit.
Closes bug #39634
44657db3e2