Rectangular selection changes on scroll

[ctrlcctrlv]

Consider the output:

SUMMARY: AddressSanitizer: heap-use-after-free ../gdraw/ggdkdraw.c:261 in _GGDKDraw_OnWindowDestroyed
Shadow bytes around the buggy address:
  0x0c207fff9660: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
  0x0c207fff9670: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fa
  0x0c207fff9680: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
  0x0c207fff9690: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fa
  0x0c207fff96a0: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
=>0x0c207fff96b0: fd fd fd[fd]fd fd fd fd fd fd fd fd fd fd fd fa
  0x0c207fff96c0: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x0c207fff96d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa
  0x0c207fff96e0: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x0c207fff96f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa
  0x0c207fff9700: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd

Create a bunch of lines containing anything, create the output, then more lines, so you can scroll.

Ctrl-Alt-Shift to create a rectangular selection containing:

fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fa
fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fa
fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
fd fd fd[fd]fd fd fd fd fd fd fd fd fd fd fd fa
fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa
fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa
fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd

When you scroll down, the selection will change to cover the beginning of the lines. I think this bug is due to #2284, but did not get time to test before that commit.

[kovidgoyal]

Quite possible that PR caused it will look into it when I have a moment.
I really need to create a testing system for selections. It is
sufficiently hairy and full of corner cases.