Author: William Gerbert

Is it somehow possible to "check" in the WM_WindowPosChanging message if the window 
is resized?

Answer:

In this message, Msg.WindowPos^.flags holds the flags which were originally used 
with the SetWindowPos, etc. functions. The window will change its size if 
SWP_NOSIZE is not contained in this flags, and either cx or cy are different, 
compared to Width and Height values of the form:
1   
2   procedure TForm1.WMWindowPosChanging(var Msg: TWMWindowPosChanging);
3   begin
4     if (Msg.WindowPos^.flags and SWP_NOSIZE = 0) and ((Msg.WindowPos^.cx <> Width)
5       or (Msg.WindowPos^.cy <> Height)) then
6       Windows.Beep(1000, 20); {sizing}
7   end;