This is what I use. It appears to disconnect, but also doesnt seem to be an issue. My machines re-organize instantly.
It's relatively uncommon and not always implemented super well, but it's a requirement for any DP KVM to be not super annoying IMO.
There was one particular KVM brand that was supposed to do it well whose name is escaping me now :/. I was looking at buying one in ~ May 2020 for obvious reasons, but they were on super-backorder (also for obvious reasons), so I never got around to it. IIRC they were about $500 for a 4 input/2 output version, so not cheap.
https://store.level1techs.com/products/14-kvm-switch-triple-...
* The total cable length is important, both between the host / KVM and the KVM / monitor, as well as any daisy chained displays you have. I had to use certified cables to get everything working reliably with my setup.
* There's a weird interaction with BIOS power on. The boot display drivers I have freak out if they aren't the active display and fail. I solve this by switching the KVM before I turn the computer on. After everything is booted into an OS, it works fine to switch.
* Power supply quality is important. I had some issues before I made sure the power supply was reliable.
KVM switches are just inherently difficult little devices. I haven't had issues since I got it working though.
Do you have an AMD GPU by any chance? I have the level1tech 2-head DP 1.4 KVM, with an AMD RX 560 on a Linux host, and after updating to kernel 6.4 recently my computer now boots fine without a monitor attached.
I had a similar issue where a display had to be _on_ and _connected_ (i.e: active on the KVM) at boot time, or the GPU wouldn't work at all. I could get in via SSH, so I tried various amdgpu recovery options, poking the device to reset it, reloading the kernel modules, etc., and never had any luck. I just lived with the quirk. It was problematic because if you left home with the KVM selected on the Windows guest, and needed to reboot the Linux host remotely, you'd come home to a non-functional Linux desktop.
I ended up ditching it on eBay at a significant loss for a $30 usb switch and just switch monitor inputs manually. Far cheaper solution and way less fussy.
Bonus for adding 'glide and switch' functionality, so you can move the mouse to the edge of the screen and it would jump the input to the next display in your layout. It's like a hardware version of Synergy.
Very finicky device, but if you don't touch it - and you don't use any of the shortcuts - it works.
Synergy made me manually configure my monitors by dragging little boxes around in a window, would frequently refuse to connect, would spontaneously disconnect, and repeatedly mangled my config such that I had to keep manually configuring it over again.
With ShareMouse, it was "open a copy of it on each machine, slide mouse in direction of next machine, then optionally enable encryption (to prevent other users' instances of ShareMouse from being able to attach)".
https://store.level1techs.com/products/14-kvm-switch-dual-mo...
The startech one I have is alright... But Apple computers absolutely hate it and frequently refuse to display, and sometimes Windows gets stuck and USB devices stop working. Strangely enough... Linux doesn't ever have any problems with either display or input. A rare win, but fine by me.
I have a bunch of them and I like them pretty well, but getting a bunch of computers all plugged in turns out to be a bit of a nightmare, especially when you need some long-ish cable runs or you are daisy-chaining devices (e.g., multiple KVM switches, adding USB hubs or Thunderbolt docks, etc.).
The Level1Techs KVM switches don't meet GP's criterion for hotplugging behavior, unfortunately. Switching between devices is just an unplug and replug for them.
Like you, I've found that macOS and Windows don't handle hotplugging monitors well, but Linux desktops (in my case, KDE Plasma) consistently do the right thing and don't require a repeater to lie to them about monitors always being plugged in.
FWIW, I don't get the 'Apple computers just refuse to work' issue with any of my L1T KVMs.
My work intel macbook worked great on it for like a year once I got a high quality usb-c -> displayport cable, but an os update borked it... though it's definitely the mac that's the problem as it also has problems sometimes with just strait to the monitor too (on the other hand 49" ultrawide is pushing the bandwidth near it's limits).
My personal arm macbook has always worked great on it with even a crappy usb-c -> displayport
my windows desktop also always worked great on it.
One of the things I've learned too late is that using DisplayPort (and maybe HDMI, idk) anywhere near its bandwidth limits is not worth it for me. Having to think about cable run lengths as well as cable quality and peripheral quirks and internal Thunderbolt dock bandwidth allocation and so on and so on just fucking sucks.
It'll probably take until the next/latest (2.1) generation of DisplayPort propagates before using multiple monitors with specs similar to my current ones (high refresh rate and HDR, but not even HiDPI) isn't painful, cumbersome, and finicky.
I probably won't be able to use them by then anyway. Ugh.
Rextron [1] is the actual ODM. They don't do any direct to consumer sales, though. That's why L1 / Startech / other "brands" sell them on amazon and the like.
Last I spoke with the L1 guy, they were still having some issues with the high speed USB-C switching chips on the 1x4 "all USB-C" model that he's got a wait-list for.
Another commenter posted that the L1Techs KVM are Rextron devices. The Startech switches are rebranded ConnectPro KVMs.
Everything else seemed to handle it fine with Linux being especially unphased, as usual.
Also, even if the drivers are solid, they take longer to renegotiate with a monitor that was removed and plugged back in compared to one they think was always there, which matters if you switch back and forth frequently.
Lastly, sometimes the OS doesn't put things back they way they were when you plug a monitor back in. If you have a laptop which has a lower resolution display than the external monitor, you'll often return to find all the windows shrunk to fit the laptop display. Not an issue if you run everything full-screen, but annoying if you tile windows.
They all try to be too smart.
As a matter of fact, I usually use a monitor switch of some sort, then use mechanical USB switches - one for keyboard, one for mouse. That seems to be the only way to get mouse and keyboard to work well (basically just a hardware connection, no smarts)
I'm still using HDMI because I like to share my home multi-monitor setup between my personal machine and my work laptop, and the KVM switches are able to fool the PCs into thinking the monitor are always connected. Years ago I tried a Displayport switch, but it could not -- I assume because if the greater sophistication of the Displayport protocol.