I know I know, it's not supported, not allowed, totally impossible because the very essence of Gnome 42. Plus, there are workarounds via the gnome-shell-extensions, namely the 'floating-dock + just perfection' combo. End of the story, right? Right?
Wait a sec, so why it is working flawlessly in Ubuntu then?
Good question.
Ubuntu GNOME is an official flavor of Ubuntu, featuring the GNOME desktop environment.
Next question.
You kind of answered your own question. Ubuntu does not ship stock Gnome, it comes preconfigured with tweaks and extensions the developers chose to feature.
Before you suggest Garuda offer an opinionated Gnome spin like this, bear in mind Ubuntu can run a frozen version of Gnome so they don't have to worry about an upgrade breaking all their tweaks and extensions, which a rolling-release distro obviously does not do.
Ok this is a reasonable answer even if tbh I was looking to the technicalities that make such a trivial semi-aesthetical tweak so radically impossible to implement. And tbh I find it hard to understand where the improvement is in such a feature. I mean, it’s literally 30 years everybody uses a taskbar at the bottom of the screen (for $REASONS), now it becomes a ‘no more’ thing (or rather a ‘one click n zoom-out thing + one more click and zoom-in thing if you want to come back to where you were’)) because screw it, we just won’t give you that?
I understand it’s a gnome thing so I should ask them but it shouldn’t something out of the world to ask for I guess.
Ubuntu uses dash to dock cosmic it can be found on the extensions .org site its not even a ubuntu app by the way
I'll chack that, thanks. I'm finishing setting up kali. Just switched to gnome (42.2 btw) and guess what... beautiful auto-hide dock we all dig ^^
They kept GNOME 41, so older extensions will still work. Nope?
If it works in GNOME 42 there then it works in GNOME 42 everywhere, which means you were doing something differently before.
Like… using xfce? 
On Ubuntu it says it’s running on gnome 42 (doesn’t specify the subversion) but maybe you meant to say they kept the same code of gnome 41? Not sure I understood tbh. Anyway I installed the floating dock for gnome 42 and it works, so I assume it’s gnome 42.
Hm. There were loads of news stories prior to 22.04 being released that said they were retaining 41 (or at least elements of 41) for compatibility reasons.
Looking at the package list they do have various bits of 42:
https://packages.ubuntu.com/search?suite=all&searchon=names&keywords=gnome-shell
Although they are also heavily patched:
https://changelogs.ubuntu.com/changelogs/pool/main/g/gnome-shell/gnome-shell_42.0-2ubuntu1/changelog