Panning to the bottom right corner (or top left corner) switches
to scroll mode and scroll the page forward (resp. backward) 1/3rd
of the screen.
One has to pan out of the corner to continue selection.
Panning again to that corner scrolls another 1/3rd of screen.
Page mode is restored when highlighting or dismissing the highlight
dialog, and a little marker is shown at where selection was started
so one does not get lost after all that scrolling and restoring.
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