a KVM come to mind except for the ability to have "a window that can dragged around on screen, resized, minimized, etc.". there are some video input cards that could work but would be limited to one or two specific input types. so would still need like a KVM to switch/adapt between multi machines..sounds like a lot of money to invest in some specific parts.....anytime you have to use adapters and special input conversion you loose video quality but if that part is not an issue. As an example of some capture cards specific to PC input are the INOGENI at $300, the Black Magic at $120, Hauppage Pro at $150 but they all support just one or two inputs from quick look over. Am sure someone more knowledgable will better info. Just hope you have deep pockets 🤣

The Datapath VisionRGB-E1 cards I have make this possible, depending on capture software.
The VisionRGB-E2 cards have two inputs each so this would make for even more connected machines.
There is a thread about screen capture in this forum.

Of course you can't control a PC with those. I guess a KVM switch is the best solution here.

Thanks, I'll have a look at some of those cards. VisionRGB-E2S sounds promising - it even mentions using VLC as a capture window for streaming.

I have a desk with a second keyboard and mouse that pulls out until my main set so I'll likely just use that with some sort of KM unit.