Determines which pop-up window owned by CWnd was most recently active.
Identifies the most recently active pop-up window. The return value will be the window itself if any of the following conditions are met:
The window itself was most recently active.
The window does not own any pop-up windows.
The window is not a top-level window or is owned by another window.
The pointer may be temporary and should not be stored for later use.