I didn't know about this. For others, it's called tmux control mode, and integrates tmux within a terminal emulator for things like split panes and tabs. It requires terminal emulator support. Here's some documentation: https://github.com/tmux/tmux/wiki/Control-Mode

Are there any Linux terminal emulators that implement this? So far the only I've encountered is macOS's iTerm 2 and it's brilliant but mac-only.