bug#62447: 30.0.50; xref default behaviour changed after a desktop-read

[Juri Linkov]

> I'm using desktop-save-mode and since sometimes when I do a M-. on
> function call in a C buffer, Emacs says:
> user-error: No definitions found for: scroll_command
>
> If I start without desktop-save-mode on, this works as usual: Emacs asks
> me if I want to read the TAGS table.
>
> So it may be that some mode loaded by some file opening mess up with
> xref default behaviour... I don't know where to start to track this
> down.

I guess this is because the default value of `desktop-globals-to-save'
contains:

    tags-file-name
    tags-table-list

and the global values for these variables make no sense.
They should have project-local values, but project-local values are
not saved to the desktop file, only global and buffer-local values.