dollarmath: inline $…$ spanning a newline parses inconsistently (second span leaks as raw text)

[mmcky]

Summary

When a paragraph contains two inline math spans that each cross a hard line break, the dollarmath inline parser handles them inconsistently: the first span parses as math, but the second one fails to parse and leaks the raw $…$ LaTeX as visible text (including the $ delimiters).

Minimal reproduction

test.md:

Now note that under the policy $\sigma
\equiv 0$, where the firm chooses never to exit, we have $T_\sigma v = \pi
+ \beta Q v$. Done.

Build with myst build --html and inspect the page AST.

Expected

Both inline spans render as math. LaTeX treats a single newline inside $…$ as a space, so $\sigma \equiv 0$ and $T_\sigma v = \pi + \beta Q v$ should both be math nodes.

Actual

The first span parses; the second leaks as literal text. The paragraph's children come out as:

node type	value
text	Now note that under the policy
inlineMath	\sigma\n\equiv 0
text	, where the firm chooses never to exit, we have $T_\sigma v = \pi + \beta Q v$. Done.

The second $…$ is emitted as a text node, so the raw LaTeX (delimiters included) is shown to the reader in the rendered HTML.

Environment

Observed on myst v1.10.1 (QuantEcon qe-v8 fork). Reproduces with the minimal test.md above in a stock book-theme project.

Context

This surfaced while converting the QuantEcon Dynamic Programming Volume I LaTeX source to MyST, where inline math frequently spans source line breaks. The converter side is being hardened to collapse those newlines, but the inconsistency here (one span OK, a structurally identical one leaking) looks like a genuine parser robustness bug worth fixing upstream too. Reported downstream in QuantEcon/book-dp-public#30.