From 5ef74d563991cdf25074ef802f4c5bdde7553f5d Mon Sep 17 00:00:00 2001
From: Elian Doran <contact@eliandoran.me>
Date: Fri, 17 Apr 2026 07:35:17 +0300
Subject: [PATCH] fix(markdown): mermaid not rendering

---
 apps/client/src/widgets/type_widgets/code/Markdown.spec.ts | 5 +++++
 packages/commons/src/lib/markdown_renderer.ts              | 5 ++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/apps/client/src/widgets/type_widgets/code/Markdown.spec.ts b/apps/client/src/widgets/type_widgets/code/Markdown.spec.ts
index 3153e8b7a0..aeb1aa1f41 100644
--- a/apps/client/src/widgets/type_widgets/code/Markdown.spec.ts
+++ b/apps/client/src/widgets/type_widgets/code/Markdown.spec.ts
@@ -70,6 +70,11 @@ describe("renderWithSourceLines", () => {
         expect(html).toContain('<aside class="admonition note">');
     });
 
+    it("preserves the `mermaid` fence language so the mermaid rewrite can match it", () => {
+        const html = renderWithSourceLines("```mermaid\ngraph TD;\nA-->B;\n```");
+        expect(html).toContain('class="language-mermaid"');
+    });
+
     it("produces math-tex spans for inline math", () => {
         const html = renderWithSourceLines("Energy: $e=mc^2$.");
         expect(html).toContain('<span class="math-tex">');
diff --git a/packages/commons/src/lib/markdown_renderer.ts b/packages/commons/src/lib/markdown_renderer.ts
index 7f0b070b3c..865880e190 100644
--- a/packages/commons/src/lib/markdown_renderer.ts
+++ b/packages/commons/src/lib/markdown_renderer.ts
@@ -155,7 +155,10 @@ class CustomMarkdownRenderer extends Renderer {
 
         text = escapeHtml(text).replace(/&quot;/g, '"');
 
-        const ckEditorLanguage = getNormalizedMimeFromMarkdownLanguage(lang);
+        // `mermaid` isn't in the MIME dictionary, but CKEditor/Trilium's
+        // mermaid rewrite specifically looks for `language-mermaid`, so
+        // preserve the fence language verbatim instead of falling back to auto.
+        const ckEditorLanguage = lang === "mermaid" ? "mermaid" : getNormalizedMimeFromMarkdownLanguage(lang);
         return `<pre><code class="language-${ckEditorLanguage}">${text}</code></pre>`;
     }