Shame on me for not having discovered / reasoned this out earlier.
I finally managed to get the axis to respond to a mouse click. Come to think of it, it wasn't so difficult and in line with the "philosophy" that no "building block" is to be built by the user in appDesigner. So, here's what I did, in case it helps.
First, in the Component Browser, I tried 'right-clicking' on the axis name and got some contextual menu. Oooh!
Second, as you can see in the screen capture, in the list of options was a 'familiar' Callbacks option! With the option to Add a ButtonDownFcn callback! Wow! I thought I was in business.
I selected the option and immediately, Matlab created the skeleton code to handle the event:
% Button down function: TroisD
function TroisDButtonDown(app, event)
a = 33;
b = 35;
end
Naturally, I add my usual "a" and "b" variables and set a breakpoint on the "b" assigment statement.
I pressed Run, selected some image and clicked inside the scatter3 drawn inside my app.axis and was immediately taken to my breakpoint!
So lesson learned :-)
Now onto some interesting stuff :-)