Why did you dislike `tmux_pretending_screen`?

well, it is no that reliable as it can be thought. It is some cases where users pass `TMUX` variable to shells to avoid some bugs. By the way, there is also `TMUX_PANE` variable, which have lesser chance to be used for such hacks, but it is anyway very bad way to detect because of the said reason 😼

we set this on line 1442 and then set it again on 1449. Would be clearer if the condition on 1447 guards lines 1441-2 with if/else.