That's the main problem with tmux in my experience, whenever I open an issue or a feature request on tmux github, the devs say it's a terminal issue. I then open the same issue on the terminal github and the devs there say it's a tmux problem.

Perhaps a solution could be combining a terminal and tmux into one product, so there would be no more excuses, however I don't think core devs are interested in getting a proper mouse support working in tmux.