https://github.com/goffioul/QtHandles/blob/master/src/gl-select.cc

The WindowButtonFcnXXX handlers are invoked whenever the mouse action occurs in the figure window, whatever is actually under the mouse pointer. The BtnFcnXXX handlers of the figure are only invoked for the corresponding mouse actions, but only when there's nothing else under the mouse pointer.

The WindowButtonFcnXXX properties only exist for figure objects.  The ButtonFcnXXX properties exist for any (primitive) objects.

After entering the commands below,

graphics_toolkit fltk
close all
buttondownfcn = @(obj, event) disp (obj);
set (gca (), "buttondownfcn", buttondownfcn)

The axes handle should be printed to the command window each time the new axes object is clicked on.

I don't know if this is only a MacOSX problem or if it exists for all OSes.