> math-stc.mkvi:
>
> \unexpanded\def\math_stackers_make_double#top#bottom#category#codepoint#codeextra#text%
>       ...
> -     \ifx\currentmathtext\empty
> +     \ifx\m_math_stackers_text_middle\empty
>         \setbox\scratchboxthree\emptyhbox
>       \else

Thanks for looking into this, Wolfgang.

I made this change to math-stc.mkvi in the 2014.05.17 beta, then remade
the MkIV format with

  mtxrun --selfupdate
  mtxrun --generate
  luatools --generate
  context --make

Then I ran context on my test file (with 30000 as the \dorecurse count).

However, the offset underbraces remain.  I'm attaching a image from
p. 2128, by which point the offset is 20 or 30pt.  I am fairly sure I am
running the newly created version, because remaking the format included
these lines:

  open source     > 1 > 256 > /home/sanjoy/context/2014.05.17-beta/tex/texmf-context/tex/context/base/math-stc.mkvi
  loading         > ConTeXt Math Macros / Stackers
  close source    > 1 > 256 > /home/sanjoy/context/2014.05.17-beta/tex/texmf-context/tex/context/base/math-stc.mkvi

and the log output from running the minimal.tex shows that the the fmt
is indeed dated 2014.7.3:

  mtx-context     | run 1: luatex --fmt="/home/sanjoy/context/2014.05.17-beta/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en" --jobname="minimal" --lua="/home/sanjoy/context/2014.05.17-beta/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en.lui" --no-parse-first-line --c:currentrun=1 --c:fulljobname="./minimal.tex" --c:input="./minimal.tex" --c:kindofrun=1 --c:maxnofruns=8 "cont-yes.mkiv"
  This is LuaTeX, Version beta-0.79.1 (TeX Live 2014/dev) (rev 4972) 
   \write18 enabled.
  open source     > 1 > 1 > /home/sanjoy/context/2014.05.17-beta/tex/texmf-context/tex/context/base/cont-yes.mkiv

  ConTeXt  ver: 2014.05.17 23:46 MKIV beta  fmt: 2014.7.3  int: english/english


-Sanjoy