Tmux grid border in second session window

Question

I attached to an existing tmux session from a second iterm2 window on a second screen.

tmux new-session -t matt

For some reason my session has a dotted border around the window. Why is this happening?

Answer 1

This is very similar to this question: Is there any way to redraw tmux window when switching smaller monitor to bigger one?

One way to prevent this (the dots around the "useful" portion) is to detach all other sessions when you attach:

$ tmux attach -d -t matt