When the pointer reaches the edge of the view during a drag (or leaves
the view entirely, which mouse capture makes possible), keep scrolling
and extending the selection: slowly on the innermost edge row, faster
on the outermost row, and very fast beyond. Scrolling starts after a
short delay so that a drag merely passing near the edge doesn't scroll.
When the view loses focus mid-drag (e.g. because a popup appeared),
cancel the autoscroll and the mouse capture.
Stefan Haller
b682fb7635