Relevant IRC discussion:

- https://irc.koha-community.org/koha/2024-02-26#i_2538655
- https://irc.koha-community.org/koha/2024-02-27#i_2538955

tl;dr

- https://irc.koha-community.org/koha/2024-02-27#i_2539035

I had this set in my database from 12 years ago when someone had a custom theme for our Koha:

MariaDB [koha_rsc]> select * from systempreferences where variable='opacthemes';
+------------+----------+---------+--------------------------------------------------+--------+
| variable   | value    | options | explanation                                      | type   |
+------------+----------+---------+--------------------------------------------------+--------+
| opacthemes | prog_rsc |         | Define the current theme for the OPAC interface. | Themes |
+------------+----------+---------+--------------------------------------------------+--------+

When upgrading from 21.05 to 21.11, this caused the return value of `sub get_xsl_filename` in `C4/XSLT.pm` to be "/usr/share/koha/opac/htdocs/opac-tmpl/prog/en/xslt/MARC21slim2OPACDetail.xsl" instead of "/usr/share/koha/opac/htdocs/opac-tmpl/bootstrap/en/xslt/MARC21slim2OPACDetail.xsl" when `OPACXSLTDetailsDisplay` is set to "default". This caused the parts of the page where the XSLT output should be to be completely empty. No errors were shown in any of `/var/log/koha/*/*.log` when reloading those pages.

Workaround: Put full paths like "/usr/share/koha/opac/htdocs/opac-tmpl/bootstrap/en/xslt/MARC21slim2OPACDetail.xsl" in the `OPACXSLT*Display` preferences instead of "default".

Manual fix:

1. Run this query: `update systempreferences set value = "bootstrap" where variable = "opacthemes";`
2. Restart everything: `systemctl restart memcached && systemctl restart apache2 && systemctl restart koha-common`

Proposed permanent fix:
- Have a fallback value for XSLT files when it does not exist on the disk AND show a warning in the logs. OR: Don't have a fallback, and throw an error and crash the program with a sensible error message.
- Show a warning somewhere when "opacthemes" contains an invalid value.

Feel free to close this if it's already fixed in a later version.