[SOLVED] Blank screen with Xfce4 after HDMI KVM switch

[vm666]

(for whatever reason, I misplace this message. Sorry!)

I have a small 4 ports HDMI+USB KVM. One machine is a mini PC with a Tiger Lake proc (Intel 11800H) and an integrated GPU.
I had some freeze or crash issues, so I changed many things on this machine. Finally I opened the machine and changed the thermal paste, it looks better now.
Anyway, for about one or two weeks, it has been impossible to use the KVM: when I come back to the miniPC, the screen is blank. The X11 system is still running, I can dump the screen with xwd, but there is no way to restore it. This is driving me nuts, it used to work before.
I booted on old kernels and I have the same problem, so I guess this is a userland problem.
Most of the time, I can keep the X server with SysRq K and log in again, but that is still a big pain in the back.
There is nothing in dmesg when I switch and switch back but the disconnection and reconnection of the USB keyboard and mouse. I tried enabling i915 debug and it shows that the HDMI cable is enabled too when I switch back.
elogviewer tells me that there was an minor update of x11-base/xorg-server in January. I tried downgrading it, to no avail.
I tried booting with nomodset and/or acpi_osi=Linux. No improvement.

Something interesting: if I just stay on the x11-misc/slim display manager login screen, I can switch back and the screen is still here. So I tried Lxqt and GNOME, both work! The problem seems to be with Xfce4.
Has anybody seen that? How can I debug that?

[vm666]

I still don't know why the behaviour changed in such a toxic way but I found a workaround

In the settings manager: go to Hardware / displays, Advanced, add here, add a profile
When a display is connected = Show dialog
First try to enabled a profile = When a display is connected
Show popup to identify display = True
Apply, close

[shimbob]