Allow "markers" that simply retain their position

I would like to specify a certain position in my LaTeX source and see where that position ends up in the generated math. (I'm trying to display a cursor within the math output). At the moment, I cannot see a way to achieve this.

This use case could be made possible by changing both the input and output jaxes to allow for "marker" symbols that do not affect processing, except that they retain their position through all processing stages.

I tried using arbitrary characters for this (and finding them later in the generated HTML), but they do affect processing. For example, when I use the marker |, the output of f|_1 is spaced differently from that of f_1 (even after subsequently removing the |).

The proposed change would allow measuring the proportions of generated math, which would enable applications adding custom annotations, cursors, tooltips etc. to it.

The issue names no files, tests, or entry points; begin by tracing how MathJax converts LaTeX input into generated math output. Define how marker symbols retain their positions without affecting processing, including the input and output jaxes. Done means positions can be measured for cursors, annotations, tooltips, or similar overlays without changing math layout.