Using tmux causes unexpected commands "1;2;"

[Yue Yu]

When I try to launch tmux inside the Upterm, it gives me "1;2;"

[Dom Hastings]

I was also able to replicate this, it looks like some sort of escape sequence: \x1b>1;2; not sure if that's a tmux thing that helps identify, or just an unhandled escape sequence by the app. Googling that sequence didn't result in anything useful, I did see a few notes around tmux specific escape sequences, but couldn't see a reference to this anywhere.

Hope that helps clarify the problem here!