While editing @Ruy's answer to How can one define a kind of "determinant" on a reduced group $C^*$ algebra?, I tried to insert an
\eqref to the equation labelled
star3 in the main post. Although equation labels seem to be required to be unique within a page (because they are implemented by anchors; for example
star3 is at
https://mathoverflow.net/questions/347474#mjx-eqn-star3), the MathJax parser was baffled by my reference to
star3, since it was not defined in the post itself. (I'm not sure if the reference would have rendered correctly if I submitted the edit; I know that sometimes the preview renders differently from the posted content.)
It seems quite likely that this is not a bug—it's of a piece with the requirement that macro definitions only extend through the post that they define—but, since equation labels really are global (per-page) objects in ways that macros are not, perhaps it is not the correct design decision; so I guess I'm calling this a feature request for the capability to refer in one post to an equation labelled in another post on the same page. It is not a particularly urgent request since it can currently be solved by manually editing in the appropriate anchored link, but that is fragile in the face of possible future changes to the implementation of equation references.
I couldn't find any other relevant tags, but please feel free to edit them in if you know any.