Compatibility decomposables that are not compatibility characters

[Sławomir Osipiuk]

On Thu, Feb 17, 2022 at 8:36 PM Ken Whistler via Unicode
<unicode at corp.unicode.org> wrote:
>
> The addition of the *canonical* decompositions for 2000 and 2001 was a
> Unicode 2.0 innovation, when it became clear that nobody could come up
> with a convincing distinction between an "EM QUAD" as a space character
> and an "EM SPACE" as a space character.

While following a different trail a couple of weeks ago I came upon
this proposal:
http://www.unicode.org/L2/L2019/19115-fwsp-usability.pdf

While the proposal itself is a non-starter due to stability reqs,
Marcel Schneider makes the case that the QUADs were originally meant
to allow line breaking, while the adjacent SPACE characters should
have been non-breaking. That would have been the "convincing
distinction", if it had been implemented that way.

Sławomir Osipiuk