Useful but dumb tidbit about Flatpak.
Over the years, I migrated my system from OpenSolaris to FreeBSD and then back to Linux thanks to the now-defunct Project Trident.
Due to Trident, I migrated to Void Linux (which remains my favorite distribution so far) and happily chugged away for several years with my $HOME in '/usr/home'. Flatpak? Seemed somewhat broken and I was never hugely concerned because I never truly had reason to use Flatpak.
Errors such as
Not sharing /usr/home with sandbox: Path "/usr" is reserved by the container framework
... never really bothered me. Instead, I used xchroot when needed.
I never realized how simple it would be to fix the issue. It appears that Flatpak expects $HOME to be in /home, which is essentially the current Linux and Solaris standard.
There's other ways of fixing this problem, other than moving the home directory location, but... I figure this will probably save me headache down the line.