And now a new feature implemented in bug#53981 allows to replace
the unreliable line above with using outline-search-function:
Now pushed to master in the commit 3573ebfa6d (it seems this is
backward-compatible since it only sets buffer-local variables).
Nice.
Do you plan on adding an outline-[minor-]mode command to hide/show by
regexp?
Do you mean enabling outline-minor-mode in the xref buffer?
Currently to enable it, the users need to add to the init file:
(add-hook 'xref-after-update-hook 'outline-minor-mode)
that is not too self-evident. Maybe a new option could help?