On macOS it is pretty obvious to have someting looking good at first.
But on GNU/Linux I always struggle to get things neat when making an app.
Is this because i use an old system (i.e. Ubuntu 20.04 LTS)?
Is this supposed to be fine on laters/others (and which one)?
What is the distribution used by JUCE devs?
Or is it because making GNU/Linux app is not the top most used part of JUCE?
Am i the only one dude to bear that pain? 
Welcome to the Linux Zoo
I’d said that if your software is open then use what you want, fixes will arrive soon or later from the community (I use openSUSE Tumbleweed); if it’s closed/commercial than Ubuntu 18 LTS or CentOS 7 are still the lowest supported “standards”.
I use several distributions, and i have no real preferences.
At home i daily use Ubuntu and Manjaro.
My question is more which one is fully (the most) supported by JUCE (i mean while testing and developing)? Specially for native GUI support. 
I don’t know which is “officialy” supported. I think your problems could be related to the ubuntu’s patched Gnome. I’d to check these under other DE and WM.
Since all my stuff is WIP i don’t want to test all the available flavors of Linux for each commit.
I use Gnome for Ubuntu (and also for Manjaro) because AFAIK it is the default DE on current (and next) LTS.
I assume that it is also a problem for the JUCE team (with limited time) to support everything.
It would be nice to have an official most supported distribution to avoid the headaches at first while developing, reporting bugs and such.
But perhaps that usual Linux distro wars would restraint that.
My 2 cents.
EDIT: By the way it seems here that Ubuntu LTS is targeted.
That info might be correct just for 20.x LTS.
Ubuntu 18 has an old CMake. Ubuntu 16 doesn’t even have a proper g++.
Yep. I suspect the info to be outdated also.
I haven’t tried with more recent versions, but JUCE 6.1.0 compiles fine on Ubuntu 16.04.
