If I run
tmux from my cygwin bash shell it prints
1;2c on my prompt after tmux launches. If I attach to a session I had open earlier using
tmux attach-session it does the same thing. If I had vim open in that session it will act as if I had typed
1;2c in vim.
I made my
.bashrc empty so I know it's being caused by tmux, not my bash settings.