bug#56171: Request for marking DEF entry when using completing-read

[Drew Adams]

> It would be valuable to mark the default entry
> when users scroll through the options defined
> for `completing-read` whenever a DEF is supplied.

I'm not sure what you mean.

Is this about highlighting in `*Completions*' a
completion that's the DEF?

Is it about selecting (as the active region) the
DEF value when you insert it with `M-n'?

Something else?

And is the request to be able do whatever-it-is
_optionally_ or always?