What You Split Was Not the Screen

Split tmux left and right, then top and bottom. Server on the left, a shell at the top right, the log at the bottom right. One curl moves all three.

Splitting did more than divide the screen. Every pane you make gets a pseudo terminal of its own, with a separate shell started on it. The manual says it plainly: each pane is a separate pseudo terminal. That is why each pane has its own working directory and history, and why opening vim in one leaves the others alone.

Move the border ten columns, and the tail inside is told its terminal had changed size. It is the same signal a program gets when you drag the corner of a window.

Not one of them knows about the split.