Old Laptops as Secondary Monitors
Windows has a built in "project" functionality where you can use any other windows device as a secondary screen - not only it's super easy to use(press win+p, select the device, done) it's also very very smooth, and the other device appears as a native secondary screen in all settings panels.
The only limitation here is that it's WiFi only, and only on devices with an Intel CPU as it's using Miracast underneath. But I use a £29 Tesco tablet with a crappy Atom CPU for this and it's fantastic as a portable secondary screen that works without any cables and literally takes 2 seconds to set up.
Using older laptops as secondary monitors instead of a tablet like iPad might have additional advantage of protection against potential battery problems as laptops are generally protected against constant charging issues and many older ones also function without a battery.
My iPad got its battery bulged within a year after using it as secondary display[1]. I'm not going to trust devices with battery for applications involving constant charging anymore, at least not the ones without removable batteries or provide greater control over hardware (Pine Tab?).
[1] https://twitter.com/heavyinfo/status/1302310594423345152
If any mainstream unix-like OS kept the promise that "everything is a file" (like Plan9), adding a secondary monitor from another pc could be easy like:
mount /dev/screen2 user@remotehost:/dev/screen1You could also use tools like Synergy or input director, they make it so that you can use 2 pcs with one set of keyboard and mouse. It works great and you can even game with it, very low latency. The benefit is that you have a separate pc, so you could use Windows and Linux etc.
When my employer used to sell a raspberry pi kit, we'd occasionally get people asking if they could just plug the hdmi cable from the pi into a laptop hdmi port. Because, the cable is bidirectional, why wouldn't the signal be bidirectional?
The big laptop manufacturers each have one or two mobile displays these days, e.g. stuff like the Thinkvision M14, which is basically just the display portion of a 14" thinkpad. These are USB-C only though, so compatibility is rather limited and getting these to work on something without USB-C requires about 100 € in adapters (Wacom Link - 75 €, USB-C PSU - 25-30 €).
Or you can strip out the LCD and attach it to a LCD driver board for $20.
Or... if your old laptop doesn't boot up, you can (apparently) pull out the laptops display panel, attach it to a control board and use just the display independently.
This DIY perks video was eye opening for me: https://www.youtube.com/watch?v=CfirQC99xPc .
What I'm personally looking for is the ability to create/grab virtual displays from MacOS. I'm guessing that Luna Display dongles are EDID dummy plugs that are used also to trigger GPUs to render for VFIO setups. I think those dummy plugs can also work for a Mac. Haven't tried that though.