Don't call ReleaseCapture() on any mouse-button-up event. Instead,

Don't call ReleaseCapture() on any mouse-button-up event. Instead,
only call it when the _last_ mouse button comes back up. Otherwise,
xterm mouse tracking will lose a button-up event if you press down
two buttons, move the mouse outside the window, then release them
one at a time.

[originally from svn r8425]