RE: [External] : Re: Strange whitespaces.

[Drew Adams]

> Why are both displayed as red underscores in Emacs?

I don't think the EN SPACE is displayed that way.
Not with `emacs -Q' (no init file), at least.  But
I don't have an Emacs 28 prerelease build - maybe
they changed Emacs 28 to highlight that as well.

As for a NO-BREAK SPACE, it's highlighted (with face
`nobreak-space', which inherits from `escape-glyph')
if variable `nobreak-char-display' is non-nil, which
it is by default.
___

Note that `nobreak-char-display' highlights all
no-break (aka "hard") characters, not just the
no-break space.  IOW, it highlights hard hyphens
(code point 8209) as well as hard spaces.

Vanilla Emacs doesn't let you highlight these
chars differently (e.g. highlight only hard spaces
or only hard hyphens.  (This highlighting
is low-level - it doesn't use Font Lock mode.)

But you can highlight such chars differently, or
highlight only one of them, using library
`highlight-chars.el'. 

https://www.emacswiki.org/emacs/ShowWhiteSpace#HighlightChars