Why are random characters inserted into my tmux session?

I have been running into this problem also (my terminal being terminator), and found the comment about the UTF8 mouse helped. I was actually able to continue using my current terminal program with tmux set mouse-utf8 off.

Which seems to have solved the problem. I've set this in my ~/.tmux.conf accordingly.

The entry in tmux.conf should look like this: set -g mouse-utf8 off, with the -g global switch (use the option for all sessions).

I found that the problem is with the terminal, and not tmux. Terminals need to support "utf8 mouse", and if they don't this is the symptoms experienced. Switching to putty or uxterm solved the issue for me.

I use Cygwin mintty and found I had to change the Options;

• Options-->Mouse->Default
• Click target from Application to Window.

After that I could click inside Tmux>Vim with no funny characters being inserted, and mouse wheel started working again.