Code:

type

TWmMoving = record

   Msg: Cardinal;

   fwSide: Cardinal;

   lpRect: PRect;

   Result: Integer;

end;

// Add a handler to your forms private section:

procedure WMMoving(var msg: TWMMoving); message WM_MOVING;

// Implement it as

procedure TFormX.WMMoving(var msg: TWMMoving);

var

   r: TRect;

begin

   r := Screen.WorkareaRect;

  // compare the new form bounds in msg.lpRect^ with r and modify it if

  // necessary

   if msg.lprect^.left < r.left then

     OffsetRect(msg.lprect^, r.left - msg.lprect^.left, 0);

   if msg.lprect^.top < r.top then

     OffsetRect(msg.lprect^, 0, r.top - msg.lprect^.top);

   if msg.lprect^.right > r.right then

     OffsetRect(msg.lprect^, r.right - msg.lprect^.right, 0);

   if msg.lprect^.bottom > r.bottom then

     OffsetRect(msg.lprect^, 0, r.bottom - msg.lprect^.bottom);

   inherited;

end;