It seems like there is a feature to emulate a 3 button
mouse by press-and-release Shift (or Ctrl) then click.
The cursor gets a little "2" (or "3") flag to tell it
is toggled. This is certainly useful for one-button
mice, but really annoying when you have a real 3 button
mouse and don't need emulation. I would like to see a
way to turn it off.