OpenType extensible brace debugging

When you combine writing a new math manual with development of the math typesetting subsystem, you can run into surprising buglets. A valid traditional TEX approach to putting braces over or under (a bit of) formula is to assemble such a brace from five snippets, where the left, middle and right snippet are characters and the “even” ones are rules that can stretch. An OpenType math font can have line segments that are used instead of rules. But before that assembly happens, one can first check if there are precomposed wider variants in the font. Consider the following formula elements (typeset with simply $\overbrace{i}$, etc.):