Sorry the quality is so bad. Hopefully you can see the signal blanking out as I switch between the menu and the running system. The monitor's HDMI2 input notification appears in the top-right each time this happens.
The output resolution is 1080p@60 in both the system and the menu (I tried to show this but it's hard to see the monitor's OSD).
It feels more like the SDL window(?) is being recreated as I switch between the system and the menu.
With the FKMS driver, the monitor never blanks out and switching between the running system and the menu is instant.