Synergy is a program that lets you use your mouse and keyboard across multiple computers as if they were just other monitors; it's still actively developed for new systems, but it's been around for about 20 years and the early versions for OSX and Win 9x are open-source: https://github.com/nbolton/synergy-vintage

I recently setup Synergy 1.3.1 on my G4 Powerbook running OSX 10.4 - it connects to my host mac, an M1 Macbbok Pro running Synergy 1.41.6 - the experience is not flawless, but it is shockingly good considering the huge age jump between the two. Mixed with ethernet filesharing, it's turned the G4 from being a retro curiosity into something that can actively participate in my daily workflow.

The downside is that it all falls apart if I need to boot into OS9 - which is about 60% of the time for me! It seems like it would be an incredibly helpful tool for working with retro computers for a lot of people in general; so I'm curious to know how plausible it would be to add OS 9 support to the old sources. Is it a matter of just compiling it, or does it require a custom driver of some sort to be made? And failing that, is there an alternative method of doing network input that I'm unaware of?