More of a mental note of all the misc. selection stuff I need to do. Hopefully I can get this in very soon so that we can verify that nothing was broken.

• Add testing for the following scenarios:
  • endSelectionPosition being located out of bounds
• _GetSelectionRects() clean up
  • the pre-processing part is just a bit nasty now. There should be a way to simplify it. Need to be careful with testing this though.
• Proper selection mechanics
  • when your endSelectionPosition is out of bounds vertically, the x-position should still update, but the y-position should be at the boundary
  • when your endSelectionPosition is out of bounds horizontally, the y-position should still update, but the y-position should be at the buffer boundary
  • this is particularly important for Feature Request: Scroll from selection dragging out of window #1247. As the y-position must be detected to be out of the bounds, so that's still valid. But it should be displayed as updating the x-position.