Comment 71 for bug 1643706

> The whole issue also raises the silly question: what is so security-sensitive about /tmp?

Another, non-privileged, user can write to it, so could leave a trojan (malware) in there ...

Ubuntu has /tmp/user/uid which I assume exists to work around that issue,
but chromium wont let me see /tmp/user/<my uid>/ or file:///tmp/user/<my uid>/

One workaround would be to consistently use a directory you can access through snap, such as /home/USER/tmp, instead of /tmp. (Globally) setting TMP, TEMP, TMPDIR and/or TEMPDIR may help with this ...

https://www.the-art-of-web.com/php/where-is-tmp/ describes some other "solutions" to this problem that have caused problems.