If a comment had a scrollbar, and the user clicked on the down arrow of
it, then the button remained in the "pushed" state, as the scrollbar
invalidations were not routed to the LOK client.

With this, the button gets back to its non-pushed state after the mouse
button is released.

Change-Id: Ie4ba5d0ec07229b0cfc08532e8e91ae25f7a4c9e