I didn't say you couldn't have multiple clients, I said clients and servers are the same process forked. Or did someone add distinct client/server support to screen finally? I know theres a lot of stuff bolted onto screen over the years but I wasn't aware they dropped forked servers for the tmux model...
Did a bit of digging; the first client gets forked to create the "server". The forked server then detaches and runs in the background. You're right that -x creates an entirely new, separate client process, unrelated to the OG client or the forked server.
Without -x though it works as originally described.
Edit: gnu screen 1.0 was originally released in 1987. The -x flag was released in screen 3.0 in the 90s. TIL
I want boo to be a screen replacement, not a tmux replacement. tmux gives you a whole workspace: layout, scrollback, copy mode, a status bar. screen's appeal was that it did almost none of that: sessions, a prefix key, done. boo keeps that model and swaps the emulation for libghostty so reattach actually redraws correctly.
They also compose: a boo session is just a PTY running a program, so you can run tmux inside one if you want.
Screen is 1 server to 1 client.
In screen each client session is a fork of the screen server. In tmux there's one server and many client forks iirc.