NBSP supposed to stretch, right?

[Richard Wordingham via Unicode]

On Tue, 17 Dec 2019 06:20:39 +0530
Shriramana Sharma via Unicode <unicode at unicode.org> wrote:

> Hello. I've just tested LibreOffice, Google Docs and MS Office on
> Linux, Android and Windows, and it seems that NBSP doesn't get
> stretched like the normal space character when justified alignment
> requires it.
> 
> Let me explain. I'm creating a document with the following text
> typeset in 12 pt Lohit Tamil with justified alignment on an A5 page
> with 0.5" margin all around:
> 
> ஶ்ரீமத் மஹாபாரதம் என்பது நமது தேசத்தின் பெரும் இதிஹாஸமாகும். இதனை
> இயற்றியவர் ஶ்ரீ வேத வ்யாஸர். அவரால் அனுக்ரஹிக்கப்பட்டவையான நூல்கள் பல.
> 
> The screenshot
> https://sites.google.com/site/jamadagni/files/temp/nbsp-not-expanding.png
> may be useful to illustrate the situation. Readers may try such
> similar sentences in any software/platform of their choice and report
> as to what happens.
> 
> Here the problem arises with the phrase ஶ்ரீ வேத வ்யாஸர். The word
> ஶ்ரீ is a honorific applying to the following name of the sage வேத
> வ்யாஸர், so it would seem unsightly to the reader if it goes to the
> previous line, so I insert an NBSP between it and the name. (Isn't
> there such a stylistic convention in English where Mr doesn't stand at
> the end of a line? I don't know.)

It's not widely taught in so far as it exists.  I would avoid
placingthe word at the end in wide columns, just as I suppress line
breaks in 'Figure 7' and '17 December', but I only apply it to short
adjuncts. However, I would find the use of narrower spacing somewhere
between acceptable and desirable.  Thai has a similar rule, where there
is generally no space between title and forename, but an obligatory
space between forename and surname.  To me, this is a continuation of
the principle that line-breaks within phrases make them more difficult
to understand.

> However, the phrase is shortly followed by a long word
> அனுக்ரஹிக்கப்பட்டவையான, which is too long to fit on the same line and
> hence goes to the next line, thereby increasing the inter-word spacing
> on its previous line significantly. But the NBSP after the honorific
> doesn't stretch, making the word layout unsightly.

The strategies to deal with this general problem in English are
hyphenation and abandoning justification.  In this particular case,
your text would benefit from using Knuth's algorithm for justification.

> IIUC, no-break space is just that: a space that doesn't permit a line
> break. This says nothing about it being fixed width.
> 
> Unicode 12.0 §2.3 on p 27 (55 of PDF) says:

You're assuming that TUS is a standard.  It's much more a collection of
influential recommendations.

Richard.