Author: Jorge Abel Ayala Marentes

How to known if the user has pressed simultaneously the left and right mouse 
buttons?

Answer:

The OnMouse event is declared as follows: 
1   
2   procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
3     Shift: TShiftState; X, Y: Integer);
4   
5   if you find in the Classes unit the declaration of TShiftState is: 
6   
7   TShiftState = set of (ssShift, ssAlt, ssCtrl,
8     ssLeft, ssRight, ssMiddle, ssDouble);
9   
10  So you need to test if sLeft and ssRight are present in the Shift parameter, now 
11  your code must be like this: 
12  
13  procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
14    Shift: TShiftState; X, Y: Integer);
15  begin
16    if (ssRight in Shift) and (ssLeft in Shift) then
17      ShowMessage('The user has pressed Left and Right buttons');
18  end;