\hbox overrides setting of \displaystyle inside the box

In

\hbox{$\displaystyle \sum_n$}

the inner \displaystyle is ignored. (Actually, it is not quite ignored, but its displaystyle="true" is changed to "displaystyle="false" as can be seen in the "Show Math As MML" menu item.)

This is due to a problem in the InternalMath() function in the TeX input Jax. In addition to mixing up the displaystyle, that routine also fails to group some output properly, so that

2^\hbox{x$x$}

produces the same result as

2^\hbox{x}{x}

where the second "x" is not in the superscript.