> There's only one rule: not to make backward-incompatible changes
> without a very good reason.

With further development of the search commands based on xref, more
users perceive it as a grep replacement, that however is not based
on grep mode, so this is a good reason to make xref keybindings more
compatible with grep mode.

> Given the single precedent I found, I'm fine with declaring the
> current binding of TAB obsolete and providing a replacement for it
> ('b'? 'q'?), so that we could replace it in some future version after
> 28, if that is okay with you and Juri.

Thanks, this is surely okay.  So here is the patch that does this.
It binds 'xref-quit-and-goto-xref' to 'C-j' like in icomplete mode: