> > And in icomplete-mode the closest analogy to picking one result is 'C-j'
>
> If people like it, I'm totally fine with changing the binding to 'C-j'.

I don't use xref.  Dunno whether it uses the minibuffer.

But if it does then, FWIW, IMO `C-j' in the minibuffer should
have its usual behavior of inserting a newline.

The minibuffer lets you do ordinary text editing of your
input, which can be multiline.  It should, at least.

(For the same reason, `?', `SPC' etc. should have their usual,
self-inserting behavior in the minibuffer.  It was misguided
to have them show help and complete words.  We finally, after
decades, got `SPC' to self-insert for file-name completion,
at least.)

Just one opinion.