I ran across this thread while looking for an answer to why this value changes when Microsoft Word "disappears" a window. I maintain an app that tracks the HWND values in order to do application sharing. This works well, but Microsoft Office applications often handle these in unusual ways. In this particular case, I found that if you do the following in Microsoft Word 2013:
- Open two new documents in separate windows.
- Save the HWND values for both windows.
- Close one of the two windows.
Both HWND values will, when interrogated with the IsWindow, IsVisible, etc Windows functions, appear to be normal, still visible, etc. There's no way I can find to tell that one of the windows has been closed -- except this undocumented dwExStyle value. 0x800 will be 'on' in the window that's still visible, and 'off' in the window that isn't visible any more.
(BTW, I know you're not "supposed" to save HWND values this way -- but try tracking windows for sharing without saving this value -- not so easy!)