Testing on virtualbox or vmware player seems impossible, because
these don't allow disabling the first display (VGA-1) and booting
with the second (VGA-2) only.
1. Boot machine with one screen connected to HDMI-3 (primary output).
2. Log in
3. Plug in second screen to HDMI-2:
=> primary output changes from HDMI-3 to HDMI-2
4. OSD appears: extend to right
=> Without this patch, the new screen (HDMI-2) would stay blank.
=> With this patch applied, the screen content moves to the new
second screen.
5. Unplug second screen (HDMI-2)
=> Without this patch, the background would get black, control panel
would disappear, could only be restored by restart of plasmashell
=> With this patch applied, screen content moves to the right and
works