bug#39424: auto-scrolling

[Mattias Engdegård]

> mouse-drag-region selects whole lines, so it moves the cursor beyond
> the end of the last line in the region, to cause the scrolling.

mouse-drag-region selects characters, words or lines depending on how many 
clicks were used to initiate the drag.

If I understand Konrad's request correctly, he suggests that the last line be 
partially selected during autoscrolling, to match the horizontal position of 
the mouse cursor, at least during single-click-selection -- is that right?
For mouse-drag-region-rectangle this is a clear necessity, but only a minor 
cosmetic detail for linear selection. That's why nobody bothered to do anything 
about it until rectangle-selection was added.