Main thing that's keeping me right now is copy select with mouse that stays in pane. Tmux would always cross horizontal panes so I'd have to zoom the current pane, alt + mouse select, then unzoom. Zellij I just drag over what I want and release the mouse button. I do however miss using the mouse for pane resizing though.

tmux keeps the selection to the pane for me.

Not something I make use of though, feels faster to copy using the keyboard instead.

Did you have to use a particular setting? I looked into it a few times but couldn't figure it out.

Tried it in a container with only "set -g mouse on" in the config and it works fine (v. 3.2a).

My best guess is that your terminal selection is overriding tmux.