[roger64]

Originally Posted by roger64

The manual says that to code the nbsp, I should use a0 and then type Alt+X
a0 is for me typically difficult to find even if I know:
http://www.fileformat.info/info/unic...00a0/index.htm

That's why I am still looking for the nnbsp code even if I know this:
http://www.fileformat.info/info/unic...202f/index.htm

Found.

One can type in the calibre's editor search box:
\u00a0 to look for the no-break space (nbsp)
\u202F to look for for the narrow no-break space (nnbsp)

You also can find the latter in
Insérer un caractère spécial/Ponctuation/Ponctuation générale
and bring it up to your "Preferred" using a right click.