Right after .Fl, a middle delimiter triggers an empty scope,

Right after .Fl, a middle delimiter triggers an empty scope,
just like a closing delimiter.  This didn't work in groff-1.15,
but it now works in groff-1.22.

After being closed by delimiters, .Nm scopes do not reopen.

Do not suppress white space after .Fl if the next node is a text node
on the same input line; that can happen for middle delimiters.

Fixing an issue reported by jmc@.