Hi,

In MkIV, \mframed does not place the result in the middle of the math 
axis. Consider,

\starttext
\startformula
   f(x) = \mframed{\frac{1}{2}}^{\mframed{\frac{1}{2}}}
\stopformula
\stoptext

The MkII and MkIV results are attached.

Aditya