[XeTeX] Hyphenation of "--" with tex-text mapping on

[Bruno Voisin]

I've just noticed a strange thing, which after all might be normal:  
using the Optima font with tex-text mapping, through the fontpec  
package and:

\setromanfont[Mapping=tex-text]{Optima Regular}

then inputting en-dashes in traditional TeX form as --, it turns out  
the en-dash may be hyphenated in the middle, yielding the following  
output (the input is CNRS--INRA--INSERM--Couperin):

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(note the hyphenation at the end of the first line). If now the en- 
dash is input directly as a Unicode character (so that the input is  
CNRS?INRA--INSERM--Couperin), then it isn't hyphenated:

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Is this normal TeX behaviour, or a feature of the font mapping in  
TECkit? I would prefer to stick with traditional TeX input as much as  
possible, so that if something goes wrong in a XeTeX document and I'm  
in a rush (read: last-minute panic), I can still revert to pdfTeX  
with minimal editing.

Bruno Voisin