[XeTeX] fontspec: problem with WordSpace feature

[Firmicus]

Hello everyone,

and a word of appreciation to Will Robertson for his wonderful fontspec 
package.

In a LaTeX document I defined a \newfontinstance with the "WordSpace" 
feature:

    \newfontinstance\arabicfont[Script=Arabic,WordSpace=1.7]{Lotus Linotype}

(Yes, the interword space in this font is really too small).

Now since the command \arabicfont is called many times during processing 
(esp. because of a macro that uses it for typing line numbers in Arabic 
in the margin), it happens that the rescaling of fontdimen2 is done 
again and again, growing exponentially until xetex complains that the 
dimension is too large (probably larger that the page!) and stops.

Looking "under the hood" I realized that an additional "WordSpace=1.7" 
is appended to the string of features of the font everytime the internal 
fontspec macro is called, and the fondimen rescaling is done for every 
one of them, again and again!

Needless to say, this is probably not the intended behaviour. I guess 
saving the original fontdimen{2,3,4} of each loaded font and rescaling 
against those original dimensions instead of the currently active values 
would be the appropriate thing to do. What do you think?

NB: The problem can be circumvented by setting 
\addfontfeature{WordSpace=1.7} immediately after the first occurence of 
\arabicfont in the document, but I guess this is not a really clean 
solution...

Thanks,
François

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