losing window focus when switching desktops

Hi list,

when I am using FB on my laptop with an external display mirroring the
internal one, and with the external display being slighly smaller,
window focus is lost when switching desktop.

Steps to reproduce:
Laptop display at 1366x768 
HDMI display at 1280x720
create a window (focussed)
switch to another desktop
switch back
window no longer focused.

or:
[...]
create two windows (one focussed)
switch to another desktop
switch back
the other one is focussed now.

This does not appear when the two display have identical display
resolution.

Is this a FB problem or about X?

Thank you for all hints!
Peter