>> In a multi-column layout, the keys <left> and <right>
>> wrap to the beginning/end of the completions buffer,
>> but <up> and <down> don't.  Here is a patch that supports
>> completion-auto-wrap for wrapping to the top/bottom:
>
> Ok, here is a better patch:

Pushed to master.

Now here is a patch that implements the long-discussed feature of
using arrow keys for navigating completions from the minibuffer
only when the *Completions* buffer is visible.

It's disabled by default and can be enabled by the option
'minibuffer-completion-visible'.  It works nicely for
the in-buffer completions as well: