Author: Ernesto De Spirito

How can I prevent the user from moving the form outside screen boundaries to 
guarantee the form is always visible inside the screen work area?

Answer:

We can know if a form has resized with the Resize event (OnResize property), but 
how do we know if a form has moved? Simply by capturing the WM_MOVE Windows message.

In the message event we call "inherited" to let the ancestors of TForm process the 
message. This will update the Left and Top properties that we can use along with 
Width and Height to see if the form is placed within the limits of the screen's 
work area (the portion of the screen not used by the system taskbar or by 
application desktop toolbars) and move it if not. 
1   
2   procedure TfrmMain.OnMove(var Msg: TWMMove);
3   var
4     WorkArea: TRect;
5   begin
6     inherited;
7     if SystemParametersInfo(SPI_GETWORKAREA, 0, @WorkArea, 0) then
8     begin
9       if Left < WorkArea.Left then
10        Left := WorkArea.Left
11      else if Left + Width > WorkArea.Right then
12        Left := WorkArea.Right - Width;
13      if Top < WorkArea.Top then
14        Top := WorkArea.top
15      else if Top + Height > WorkArea.Bottom then
16        Top := WorkArea.Bottom - Height;
17    end;
18  end;


The full source code of this example is available for download: 

http://www.latiumsoftware.com/download/p0020.zip

Copyright (c) 2001 Ernesto De Spiritomailto:edspirito@latiumsoftware.com
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