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Hans Hagen
anyhow, i've now defined then in a different way (ok, some more trickery had to be applied as it's one of these aspects of open type math that still inherits old stuff) so in a next version you can do this:
\setupmathstackers[vfenced][hoffset=10pt]
Tested with 2014.02.14 beta, and it is a big improvement. Again, sorry it took so long to test. If I use hoffset=3pt, it seems that some underbraces get 3-pt borders but others don't. For example, \setupmathstackers[vfenced][hoffset=3pt] \starttext \placeformula\startformula \underbrace{15\,\tf cm}_{L} \times \underbrace{15}_{L} \stopformula \stoptext The second underbrace has the 3-pt borders, but the first one looks more like a 0-pt border (though hoffset=0pt makes it have a negative border). Is it due to quantization, in that the extensibles grow in fixed-size units? -Sanjoy