>> Since we removed the rule of visibility of next-error-function buffers
>> as a criteria for deciding what navigation to use for next-error,
>> so e.g. next-error in *Occur* or *Diff* that is the current buffer
>> and a single visible buffer on the frame will still use the last next-error
>> buffer that might be e.g. *compilation* or *grep* buffer buried long ago,
>> now we need to support the same way to switch next-error navigation as
>> already is possible in *compilation* and *grep* by typing RET or ‘C-c C-c’
>> that sets next-error-last-buffer in next-error-internal among other things.
>
> Sorry, I don't understand what kind of solution are you proposing in
> user terms.  Can you please describe what the user should do under the
> proposed solution in various cases, such as if they need to invoke
> navigation in the current Occur buffer or in a Compilation/Grep
> buffer, either shown or not shown?

I propose to set the global value of next-error-last-buffer after typing
one of the following keybindings in the corresponding buffers:

in *xref* xref-goto-xref: RET
in *Diff* diff-goto-source: RET, C-c C-c, o
in ChangeLogs change-log-goto-source: C-c C-c
in *Occur* occur-mode-goto-occurrence: RET, C-c C-c,
           occur-mode-goto-occurrence-other-window: o
           occur-mode-display-occurrence: C-o

IOW, doing the same what RET and C-c C-c in *compilation* and *grep* already do.

Here is the final tested patch with these changes: