Mouse Clicks in a Drawing Area

["ldries46" <bertus.dries@planet.nl>]

I do want to draw something at a certain point in a Drawing_Area. That point 
should be derived from a mouse click.
To do this I have created a handler "On Button_Press".
I already have a draw event connected that works. I have concluded this from 
the facts I can see and from the breakpoint I positioned inside the 
"On_Area_Draw" Handler. The mouse click handler is created analog to the 
Draw handler.
After creating the Drawing Area "Sudoku_Area" the handlers are initiated as 
follows:

Add_Events(Sudoku_Area, BUTTON_PRESS_MASK);
CR := Create(Get_Window (Sudoku_Area));
Event_Cb.Connect (Sudoku_Area, "draw",
                  Event_Cb.To_Marshaller 
(On_Area_Draw'Unrestricted_Access));
Event_Cb.Connect (Sudoku_Area, "button_press_event",
                  Event_Cb.To_Marshaller 
(On_Button_Press'Unrestricted_Access));

where:

package Event_Cb is new Gtk.Handlers.Return_Callback
  (Gtk_Drawing_Area_Record, Boolean);

The compilation of this is faultless.

I do use the following handlers:

function On_Area_Draw (Object : access Gtk_Drawing_Area_Record'Class; 
Context : Cairo.Cairo_Context) return boolean;

function On_Button_Press (Object : access Gtk_Drawing_Area_Record'Class; 
event : Gdk_Event) return boolean;

Both program build faultless.
I put a breakpoint inside the On_Button_Press function at a point where it 
should always be reached.
Running the program and clicking within the drawn area that breakpoint is 
never reached and the program runs without reporting an error.
I am using Gtk 3.4 and GPS.
What do I do wrong?