An undefined code environment no longer swallows the document

[tannevaled]

An undefined environment's body is typeset with the ordinary category codes. Right
for a prose wrapper, ruinous for a code block: a lone $ — as in

path = "$write_dir/therm_comp_MPI/"

opens math mode, and the scan then runs past the environment's own \end looking
for the closing $, eating the rest of the document with it. Against main the
minimal case does not merely lose text, it fails outright:

math error … texmath: unknown command \endjlcode

The rule

The test is the imbalance itself: prose does not carry an odd number of math
shifts, code does. When the body between \begin{env} and \end{env} has one, it
is read raw and set as a verbatim block. A balanced body keeps exactly the behaviour
it had — a prose environment is still prose, its maths still rendered (both are
tested).

Two guards keep it narrow: the \end must be in the character buffer, and no
pending token list may hold an \end of its own. A captured body (a minipage, a
float, a beamer column) carries its \end in the lists, and reading the buffer
there would copy the document that follows — the mistake #214 fixed.

Measured

No corpus movement: 516 pages of error before and after, over the 157 arXiv
papers with a tectonic reference. I am landing it on the strength of the
reproduction, not of a number.

The paper that led me here (#225) reaches this failure by another route — its
jlcode bodies arrive through a token list, so the buffer path never fires — and
stays open. This is the defence, not that paper's fix.

🤖 Generated with Claude Code