Thanks everyone for your contributions and all these hints! We could add geany and gedit to this list, even if they are, like atom, rather text editors than true IDEs, but have also interesting plugins that make them really good tools.
I tried to answer thoroughly but it made quite a long post, sorry about that...
I knew most of the IDEs you told about, but took time to make a "tour" again, trying them, installing plugins etc. They are all quite excellent, some even have interesting advantages over atom and I discovered some features I didn't know they had. But still, not one of them, in my opinion, fits my needs as good as atom, very clearly, and despite its drawbacks (a bit too heavy and missing a source code browser for python).
Why I want to use FreeBSD is I find its quality simply excellent and I wish I could use it on a desktop computer.
I do not use Windows nor Mac since 12 years now and could never use any proprietary system again, I'm too accustomed now to find the door open when I want to change things the way I want to. On my desktops, I've only used linux distributions all this time. Only made one try with FreeBSD some ten years ago, but had to compile everything, it was really too long (OpenOffice, etc.). The result was interesting, especially quick, but too complicated to maintain for an everyday usage (for me, at least). Apart from that, I use a "vanilla" FreeBSD on a server since several years now, and I appreciate it a lot, especially for its security (jails!), reliability and robustness. I'm not exactly disappointed by Linux; but from the server experience, FreeBSD looks "better", cleaner, more ordered and stable, less "chaotic" to me. (Personnal opinion).
I felt GhostBSD makes the installation and settings of a FreeBSD as easy as a mainstream Linux distribution, and I'm really enthusiast about that
I feel it's an excellent work that may help people to discover and use FreeBSD.
So, to get back to the "atom question", at the moment I cannot install FreeBSD on my main desktop, because its processor is too new and unsupported yet. But I have a laptop that's a bit older, that I use occasionally, when I work away from home and I wanted to install GhostBSD on it. This could be a start and for that, I could try to use atom with linuxulator (no experience with it) or wine. Or PyCharm might do the trick for this occasional use.
Later, when FreeBSD becomes installable on my main desktop (maybe when the 11.1 is out?), I could consider installing GhostBSD on it too, but I really need a functional atom. There used to be a FreeBSD support and I don't know why it has been dropped. I noticed that there's a bounty there:
https://www.bountysource.com/issues/290 ... o-electron so maybe there's a hope to see atom run natively on FreeBSD?
Thanks again to everyone!