Weechat. runs in Cli. Supports mouse mode and is cross platform. Run it in tmux or even better on a shell. It’s superb.

I use thelounge everyday for my open source work, it's awesome. There's a demo on their website: https://demo.thelounge.chat/

Textual is the best and it's free software. (I think it's in brew cask)

Textual works well for me.

Quassel

Textual

irssi