When using tmux in Terminal.app with the TERM preference set to xterm-color causes some weird line wrapping behavior. Images speak louder than words here:
Just before the weirdness
This blank line is the weirdness, it’s really disconcerting when typing a long command, or when using readline to look through previous commands.
The weirdness continues…
But a CTRL-l will help ease our pain.
The weirdness seems to have something to do with being at the bottom of a terminal window. The problem is not exhibited at the top of an empty window.
A solution is to set your TERM value in Terminal.app to xterm.
