Block syntax-highlighting: idiomatic for reveal.js

Reveal.js doesn't properly support Pandoc's idiomatic highlighting mode,
resulting in no highlighting at all. Emit a warning and fall back to
default skylighting instead.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cderv

src/command/render/pandoc.ts

@@ -6,7 +6,7 @@
 
 import { basename, dirname, isAbsolute, join } from "../../deno_ral/path.ts";
 
-import { info } from "../../deno_ral/log.ts";
+import { info, warning } from "../../deno_ral/log.ts";
 
 import { ensureDir, existsSync, expandGlobSync } from "../../deno_ral/fs.ts";
 
@@ -1774,11 +1774,19 @@ function resolveTextHighlightStyle(
   }
 
   if (highlightTheme === "idiomatic") {
-    // Use native format highlighting (typst, LaTeX listings, reveal.js highlight.js)
-    // Pass through to Pandoc 3.8+ which handles this natively
-    extras.pandoc = extras.pandoc || {};
-    extras.pandoc[kSyntaxHighlighting] = "idiomatic";
-    return extras;
+    if (isRevealjsOutput(pandoc)) {
+      // reveal.js idiomatic mode doesn't produce working highlighting
+      // Fall through to default skylighting instead
+      warning(
+        "syntax-highlighting: idiomatic is not supported for reveal.js. Using default highlighting.",
+      );
+    } else {
+      // Use native format highlighting (typst native, LaTeX listings)
+      // Pass through to Pandoc 3.8+ which handles this natively
+      extras.pandoc = extras.pandoc || {};
+      extras.pandoc[kSyntaxHighlighting] = "idiomatic";
+      return extras;
+    }
   }
 
   // create the possible name matches based upon the dark vs. light

tests/docs/smoke-all/latex/syntax-highlighting/idiomatic.qmd

@@ -0,0 +1,21 @@
+---
+title: LaTeX Idiomatic Highlighting
+format: pdf
+keep-tex: true
+syntax-highlighting: idiomatic
+_quarto:
+  tests:
+    pdf:
+      ensureLatexFileRegexMatches:
+        # idiomatic = LaTeX listings package
+        - ["\\\\begin\\{lstlisting\\}", "\\\\usepackage\\{listings\\}"]
+        # Should NOT have fancyvrb/Shaded (skylighting output)
+        - ["fancyvrb", "Shaded"]
+---
+
+Test that `syntax-highlighting: idiomatic` produces LaTeX listings highlighting.
+
+```python
+def hello():
+    print("world")
+```

tests/docs/smoke-all/revealjs/syntax-highlighting/idiomatic-warning.qmd

@@ -0,0 +1,22 @@
+---
+title: Reveal.js Idiomatic Warning
+format: revealjs
+syntax-highlighting: idiomatic
+_quarto:
+  tests:
+    revealjs:
+      noErrors: default
+      printsMessage:
+        level: WARN
+        regex: "syntax-highlighting: idiomatic is not supported for reveal.js"
+      ensureFileRegexMatches:
+        # Should still have skylighting highlighting (fallback worked)
+        - ["sourceCode", "class="]
+---
+
+Test that `syntax-highlighting: idiomatic` emits warning for reveal.js and falls back to default highlighting.
+
+```python
+def hello():
+    print("world")
+```