You need to set ttymouse. vim doesn't know about st, so it doesn't detect the correct ttymouse value (xterm seems to work fine for st). neovim changed the TUI (and mouse) code quite a lot so I'm guessing that's the reason It Just Works™.
- [dev] [st] vim mouse not working Pickfire
- Re: [dev] [st] vim mouse not working Martti Kühne
- Re: [dev] [st] vim mouse not working Pickfire
- Re: [dev] [st] vim mouse not working Otto Modinos
- Re: [dev] [st] vim mouse not working Pickfire