Friend sent vee this KM switch, as in KVM without the V. It promises to do screen edge detection and automatically migrate to another system like Deskflow does in software. I was skeptical it'd work, especially since I have a non-standard T-shape of displays as my main PC.
And yet somehow it does. It has to somehow be aware of relative location on the screen but I'm not sure how it would even ask for that from the OS. It's a USB device.
Neat little thing though. It doesn't just switch which lines to systems are live when you switch. It's a HID proxy of sorts and is always live on all systems. That means a micron of lag but it's not bad.
Oh I see... This is how it works... There's a method of expressing screen-absolute positions in USB HID, for tablets. You express a minimum and a maximum for each axis and then when you designate absolute positions, the OS renders a location based on a ratio of current to max. It's basically percentage.
Tablets are supposed to define a maximum axis that makes sense for their resolution, but if you just ignore that and say something like... your absolute max is 10,000, you can target locations with 0.01% screen size granularity.
Which means this thing is just tracking internally my axes as a percentage and when it hits 100, switching target system and starting over at 0.
That explains why my mouse cursor feels weird on the T-shaped display layout. If I draw a "Circle" it kind of comes out an oval. Because the display ratios don't match what the device is expecting.
Weeeeird. TIL.
@trysdyn Oh, that's *fascinating*. What an odd little device! Does it have a way to lock to a particular screen for relative mouse mode?
I've been using old open builds of Synergy myself, but idk if/when those'll break, probably no earlier than when Win10's extended service ends and I can throw my whole virtualization setup out 
@trysdyn Ah yeah, work issues aside, I'm still a crotchety old lady with my XFCE on Xorg setup. I'm not looking forward to that changing, certainly!
Glad you could find a solution, even if your friend lost out on it. Hopefully they could at least get it off their taxes
@elfi Yeah if I turn off what it calls "Roaming" it feels a ton more accurate and my presumption is it begins just passing relative movements from the mouse straight through.
I was in the same bin. I've been using Synergy/Barrier for years and years but wayland doesn't support it, Deskflow is iffy on wl-roots, and my employer technically forbids Synergy and its ilk on their laptop so I'm just like "Yeah okay I should probably find a different solution"
This thing is this: https://www.store.level1techs.com/products/p/4-port-km-switch-with-usb-32-gen-1-mouse-roaming-function which isn't exactly cheap. I got lucky. A friend bought it for professional use then found it Just Doesn't Work for their use-case, got refund rejected, and sold it to vee for a song after 3 days of use.