[XeTeX] control space, emergencystretch, glue?

[ITS Mail]

Hello

I attach a zipped minimal XeLaTeX file, with two output pdfs (one using 
our local commercial font, another one with Gentium), where you can see 
how xelatex seems to ignore my attempts to introduce an unstretchable 
space using "\ " ("control space" in the TeXbook). Even reducing 
\emergencystretch to 0pt, the initial lines, having the same first 
words, will still look so different! I have tried different fonts and 
different changes in the TeX tolerance parameters; it gets a little 
better, but the cheeky stretching of the control space is still there. 
Why so? Is there a way to produce a really unstretchable space?

I also replicated this under PDFTeX, and it does not happen, so it 
seems peculiar to XeLaTeX.

Thanks in advance.

Servus,
Juan
___________
Islamic Texts Society
22A Brooklands Avenue
Cambridge CB2 8DQ
United Kingdom
Telephone +44 (0)1223 314 387
Fax +44 (0)1223 324 342
Website: www.its.org.uk
Regd. Charity 283832
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