[macros] Avoid page break between section heading and grammar snippet

source/macros.tex

@@ -660,7 +660,17 @@
 \setlength{\BnfInc}{\BnfIndent}
 \newlength{\BnfRest}
 \setlength{\BnfRest}{2\BnfIndent}
-\newcommand{\BnfNontermshape}{\small\color{grammar-gray}\sffamily\itshape}
+\newcommand{\BnfNontermshape}{%
+  \small%
+  \color{grammar-gray}%
+  % The color setting inserts a \pdfcolorstack entry into the vertical list,
+  % breaking the connection of the \penalty entry from a preceding heading
+  % with the \glue entries that precede the grammar snippet.
+  % Add a penalty here that prevents making those \glue entries page-breaking
+  % opportunities.
+  \penalty10000%
+  \sffamily%
+  \itshape}
 \newcommand{\BnfReNontermshape}{\small\rmfamily\itshape}
 \newcommand{\BnfTermshape}{\small\ttfamily\upshape}