This function extends an existing selection in the following way:
If the extension point is *before* the upper boundary of the current
selection, extend the selection upwards.
If the extension point is *after* the bottom boundary of the current
selection, extend the selection downwards.
If the extension point is *inside* the current selection, shrink the
selection such that the new size is maximized. This means we move the
*closest* start/end point from in the current selection.
This callback is *always* called, including when there has been an
error.
This is in preparation for making text_from_{clipboard,primary}
asynchronous.
If we scroll enough, we'll eventually end up wrapping around the
entire scrollback buffer. At this point, a selection is no longer
valid, so cancel it.
Note: this was very obvious when scrolling in the alt screen, since
its scrollback buffer is what you see on the screen (i.e. it has no
scrollback).
