I mean, I mostly wanted to point that out because OP prefers FOSS, but I did think at least their IntelliJ Community Edition was open-source.
But I just checked their website and it says "Free, built on open source", arguably the worst option, although I guess, it's what the MIT folks want.
I just hate how people overwhelmingly recommend proprietary editors. It's the one thing you'd think the FOSS community should be exceedingly good at.