Function join_multiline_math 

pub fn join_multiline_math(content: &str) -> String

Fold every $$…$$ span that crosses a line break onto one line, leaving fenced code blocks alone.

CommonMark reads the lines of a display formula as block syntax — Z[J] over a lone = is a setext heading, a line opening with + c is a list item — and pulldown-cmark’s math cannot cross a block boundary, so the formula fell apart into escaped text. Whitespace is insignificant to LaTeX, so the span is the same formula on one line, which pulldown-cmark reads as display math whole.