Tmux Displaying Split Borders as Dashes in OSX Lion

When I installed tmux on my main work machine iTerm was displaying the split pane borders in what appeared to be dashes as below.

At first this wasn’t too annoying, it was just a visual thing, but when switching from one pane to the next the pane border would merge into multiple lines and it looked very messy.

I scoured Stackover Flow as I normally do and started hunting and debugging. I started with my own bulletproof troubleshooting technique. It was only happening in iTerm, Terminal.app was displaying everything fine and it was only happening under my login. Other accounts were fine. I decided to try and nuke my iTerm preferences so I ended up reading the iTerm FAQ page. Below the answer that I was looking for I noticed this question …

Q: Some characters appear to be twice as wide as others, and things aren’t lining up correctly.

A: Go to Preferences->Profiles->Text and change the setting of "Treat ambiguous-width characters as double-width."

Bingo. Unchecking the “Treat ambiguous-width characters as double-width.” setting did the trick

I can’t be sure, but I suspect that tmux wasn’t using dashes, but was using the correct character. The problem was that iTerm belived this character to be ambiguous. After unchecking the box I was rewarded with my bounty.