[pcdavid]

FWIW, LibreOffice automatically inserts an actual Unicode NO-BREAK SPACE when I type ":" at the end of a word (if the language is set for French of course). If I insert an actual SPACE and then hit ":", it even replaces the SPACE with a NO-BREAK SPACE.

I'd be surprised MS Word doesn't do the same. No need for a "true" typesetting solution.

[rodelrod]

I think the point of the GP is that it inserts NO-BREAK SPACE instead of a NARROW NO-BREAK SPACE. If that's the case, it's a bug.