Theme's javascript breaking, 0 byte js.gz files, when aggregation is on in drupal 9

dblog shows the underlying issue (discovered took a while) unable to make file directories / move temporary files

User warning: mkdir(): Permission Denied in Drupal\Component\PhpStorage\FileStorage->createDirectory() (line 123 of /home/sr-dev/web/core/lib/Drupal/Component/PhpStorage/FileStorage.php)

#0 /home/sr-dev/web/core/includes/ _drupal_error_handler_real()
#1 [internal function]: _drupal_error_handler()
#2 /home/sr-dev/web/core/lib/Drupal/Component/PhpStorage/FileStorage.php(123): trigger_error()
#3 /home/sr-dev/web/core/lib/Drupal/Component/PhpStorage/FileStorage.php(73): Drupal\Component\PhpStorage\FileStorage->createDirectory()
#4 /home/sr-dev/web/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFastFileStorage.php(66): Drupal\Component\PhpStorage\FileStorage->ensureDirectory()
#5 /home/sr-dev/web/core/lib/Drupal/Core/Template/TwigPhpStorageCache.php(112): Drupal\Component\PhpStorage\MTimeProtectedFastFileStorage->save()
#6 /home/sr-dev/vendor/twig/twig/src/Environment.php(409): Drupal\Core\Template\TwigPhpStorageCache->write()

and The specified file 'temporary://filevZEVAH' could not be moved to 'public://js/js_mI0Lsjo172QVP1eO92_XhS-iYMaHr17G2QXAaJrQ93U.js.gz'.

is our temporary directory the issue?

Temporary directory /tmp

and in conclusion this was all caused by running out of disk space

The server ran out of disk space for user-uploaded files (separate from the database, which is why there were no other disruptions). May First has increased it now, so it should start working again, and will more than quintuple the size this evening. And will add notifications for this custom server, which is standard on all ordinarily-provisioned servers.

(Chris and i identified the problem after being flummoxed by completely different weirdness—caused by the same problem—when we tried to reproduce the problem on the test site.)