Previously I've been looking for a displayport KVM that would support at least eDP 1.4, edid and USB device emulation. I found only one horribly overpriced device with bad reviews so I decided to give HDMI a go instead.

I searched everywhere and I managed to find this 4 PC KVM switch by KCEVE on amazon for $75 that claims to support USB3, 4k@60Hz HDMI2.0 and EDID emulation. I connected it to my 1 windows and 2 current linux machines via HDMI and it turns out both Linux machines have a different problem while windows works flawlesly.

As mentioned the KVM switch has EDID emulation, so when it is switched to other device the devices that are off are still getting the EDID of the main monitor preventing resolutions from being changed and desktops being messed up. This works fine in windows, but my Alma Linux 9 laptop with gnome still has its internal screen go blank for ~2s when KVM is switched to it(as if it run xrandr --auto internally). I checked that EDID stays the same, but somehow the laptop detects the disconnection/reconnection and redetects the monitors. One good thing is that despite this redetection windows present on the secondary monitor do not move, but that extra 2s delay is very annoying.

The other Linux machine is an Arch desktop running i3wm therefore it has no monitor hotplug. The very weird problem that occured is: if I switch the PC on into BIOS or I stay on the linux text console I can use the KVM normally. I can switch to other PCs and back with no problem. However, when I start Xorg and I switch away, after I switch back I get no picture... until I do either of: run xrandr --auto(which detects exact same settings), disconnect the hdmi cable and plug it again(how does this do anything without monitor hotplug detection!) or restart the KVM switch.

All my cables are HDMI 2.0 and 2.1 compatible. For now I configured a udev rule that runs xrandr --auto on the arch machine when a drm event is detected. This means I'm getting teh same 2s delay on both Linux devices while the windows one has its screen appear within 0.5s.

Has anyone got any advice how to fix this? I realise not many people use Linux on the desktop and even less people use it with KVM at 4k@60hz, but perhaps someone else does and that other person has figured that out.

I gave up on KVM a long time ago, I could not find any HDMI/USB at a sensible price - what I do is use HDMI splitters, quite cheap to buy, but you do need a keyboard/mouse on each computer.

For a while I used the input switching feature of my monitor (eDP, eDP mini, hdmi) and the cheapest USB switch I could find. When I bought the usb switch I knew it was mechanical, but I thought great, maybe it'll be more reliable. No, the opposite was true. Every 3rd or 4th switching wouod not register. I'd have to switch repeatedly. Also sometimes it would crash the usb port in the pc and nothing short of a sudden restart fixed it. I even cleaned the contacts with a contact cleaner, but it made no difference. I decided usb2 devices are simply incompatible with it. Of course the minus of this "solution" was that every time I switched the monitor input other machines wouod detect it and mess up my windows.

If I was switching few times per day that could be fine, but few times per hour was unacceptable.

Now I've found this "kceve" brand hdmi switch that seems to support edid emulation properly (I'm using 3 hdmi inputs 4K@60HZ). If not for this delay when switching it wouod be a nice setup. Perhaps I need to ask on some electronics forum instead, why is it that the signal has to restart despite the pcs thinking the monitors never disconnected.

Not sure if this info will be of any good to you, but I found this whilst looking around....

https://www.ebay.co.uk/itm/394918633005 - TESMART KVM Switch

@Luk, I'm having Linux-specific issues with the KCEVE KVM. It seems to work great with Windows, but with Linux (Ubuntu and Terabyte IFL) the keyboard is unusable.
Keystrokes are missed, doubled, or turned into long strings of characters.

I thought it might have to do with my Tecware Spectre Pro mechanical keyboard, but maybe it goes deeper to the KVM.
Although it seems my issues are different than yours.
I don't have any problem with the video side - Linux and Windows displays both work fine and switch within 5 seconds.

I have a 4-port HDMI variant of this that works great in Linux (used with two different generations of Radeon cards, handles 4K60 just fine). One thing I do know about KVMs is that a lot of fancier modern keyboards (and some mice, and 'do multiple things' wireless dongles) will give you grief, because they aren't actually HID class devices to the host, a lot of them are actually USB hubs with various stuff hanging off (e.g. keyboard, RGB controller, sound controller, extra USB ports, etc) and that can cause a lot of problems for the KVM itself (it may refuse to pass the device, it may not switch as expected, etc). Some devices have manuals that explain this more clearly than others. Unfortunately finding a high quality keyboard that is just a plain old HID seems to be increasingly scarce as everyone seems to want other connectivity (like audio) or features (like RGB) in the keyboard. If your KVM has a more generic 'USB accessory' port (which will be treated like a normal USB switch and the upstream computer(s) may log the keyboard disconnect/reconnect event depending on how it handles HID emulation) that may help with many of these devices. I've had far less problems with mice, including wireless mice, for what its worth.

For pure USB switches, IO-Gear makes electrical ones that have been very reliable in my experience, and come with wired remotes, but keep in mind some applications (like games) may not like all the HIDs going away while running, so you may want to keep an extra keyboard or mouse (or perhaps one of those 'emulator' dongles that became popular during the pandemic, but I don't know much about them) connected on the side if you go that route, and use the monitor's built-in input switching.