When I scroll a document by dragging the scrollbar thumb with the
mouse, the thumb disappears after a few seconds when I don't move
the mouse, even though the mouse button is still pressed. This
might be a feature (though I think that this is unexpected when
the mouse button is still pressed, as the intent by keeping the
mouse button pressed is the be able to scroll again), but the issue
is that when I move the mouse button again, the thumb may remain
invisible and the document doesn't scroll. The thumb even remains
invisible after I release the mouse button and move the mouse, and
I need to click to make it reappear (something that is not needed
in the normal usage).