Not knowing the internals, that's what I tried to say with adding a space after instead of 1em, i.e. "calculated by the engine". If w an em is added after, it is not stretchable and shrinkable, right?
Right. But no particular space is added after '?' or '!' in French punctuation mode, TeX sets the standard inter-word glue, as it should. The only intervention by ConTeXt is the addition of the unbreakable space before those marks (and, obviously, after '«').
Thank you so much for the detailed technical explanation! So, AFAIK, I believe that the space before should be equivalent to thinspace.
You'd need to take this up with Olivier Guéry, who suggested the 1/4 em value in the first place (refer to his interventions from last Fall, e.g., http://www.ntg.nl/pipermail/ntg-context/2008/034845.html). He wrote up a small proposal on the wiki page for French Punctuation (http://wiki.contextgarden.net/French_Punctuation).
Thanks, I'll see that. Maybe I could write some detailled specs in the wiki.
Yes, please do so by extending the aforementioned page if needed. Arthur