From a shell in a GUI with sloppy focus (assuming that they are all
similar, I am using GNOME here) do

emacs -Q
M-x set-variable RET focus-follows-mouse RET t RET
C-x C-f /tmp/testfile RET
C-x 5 2    and arrange frames side-by-side
M-! touch /tmp/testfile RET

Now move with the mouse from one frame to the other and immediately
press a key.  I get a message like