Correctly handle loss of window focus

[MikeHopcroft]

Repro steps:

1. Mouse down on green circle corner drag handle to initiate drag.
2. While holding mouse down, press ALT-TAB to switch focus to another window.
3. Release mouse.
4. Press ALT-TAB to return to MasterCopy window.
5. Move mouse. Note that drag handle moves, even though mouse is not pressed.

Expect:
should exit drag mode on loss of window focus.