Button press and release in different Windows (Regions?)


How do you track a button release on a different window (region), other than
the window (region) where the button press occurred?

I would like to implement a similar multi item selection scheme, such as in
Phab for my app.

Where do you draw the selection rectangle from? I noticed that in Phab the
selection rectangle goes outside Phab main window (region).

Any source code examples would be much appreciated.