Superscripts & subscripts in CJK text
In CJK text, there is a space before superscripts and subscripts which are made by commands like \textsuperscript.
A quick solution is to redefine \textsuperscript and \textsubscript as below (prepend and append \null). But I fear its side-effect.
cf. (u)pLaTeX seem to have the same behavior. I created an issue: https://github.com/texjporg/platex/issues/82