For 2 - I think this can be achieved with scroll vertical/horizontal scroll regions (which should also boost performance a bit). I hope to implement this in the future.
When I tried to get into programming as a teenager, I opened a graphical IDE and was immediately overwhelmed by a barrage of menus, popping windows and tooltips. This led me to believe programming is not for me. In later attempts I used vim (back then that's what people used to recommend those starting out with perl) and felt right at home.
These days I still use vim (almost stock) for everything (right now mostly Rust). This is totally anecdotal, but I'm usually more productive than my teammates - mostly because of the initial hurdles of getting into a new codebase or parts thereof. I don't have anything to set up, I just open it up and grep my way to success.
Every now and then I try to open an IDE and force myself to work with it a little bit just to be sure I'm not missing anything. The visual overload and lack of advanced editing are just too much for me. Going back to vim is always a breath of fresh air.
I'd sooner use nano/notepad.exe than any sort of graphical IDE (though I prefer vim).
I'm not saying this is the best way, I definitely do not recommend it to those asking me how to get into programming, but it's definitely the best one for me.
tmux popups are not first class citizens (eg. at least in current versions they freeze the display behind them, opening a terminal inside one of them is a pain as well as opening multiple ones, not to mention resizing them or rearranging them intelligently)
We actually looked into it and found it to be the Rust allocator after all (the memory is freed after a day or two and afaict should not cause oom issues). I'm sorry for not writing this up in the issue, this was heavily investigated by another maintainer and they have not yet found the time to write this up.
Yes they are. They work out of the box as I mentioned. This is what originally helped me move from screen to tmux, so it was important to me to provide the same experience.
As for comparisons to tmux: I shy away from those. Just like I shy away from comparing chocolate cake to lemon cake. Both are awesome. Use what you like and decide for yourself.
Session detach was added 2+ years ago and tmux keybindings work out of the box. You can also remove any part of the UI you do not like and there are some pretty friendly built-in alternatives that take up less room.
That being said - use whichever software makes you happy. I'm just a stickler for facts. :)
Not accounting for tastes and preferences, but it is decidedly not true that tmux does everything Zellij does (just some very minor examples are: true floating panes, true multiplayer support, stacked panes, etc.)
As for prevalence: this very often has to do with the distributions themselves and their ability to accept certain (rust) dependencies.
Otherwise though, I always say Zellij is not competing with tmux. That tool choices should be more equivalent to clothing choices than to choosing a school for your kids. Choose what you like, there should not be religious disagreements here.