feat(mermaid): load ELK library on demand

src/public/app/services/library_loader.js

@@ -59,7 +59,16 @@ const FORCE_GRAPH = {
 
 const MERMAID = {
     js: [
-        "node_modules/mermaid/dist/mermaid.min.js",
+        "node_modules/mermaid/dist/mermaid.min.js"        
+    ]
+}
+
+/**
+ * The ELK extension of Mermaid.js, which supports more advanced layouts.
+ * See https://www.npmjs.com/package/@mermaid-js/layout-elk for more information.
+ */
+const MERMAID_ELK = {
+    js: [
         "libraries/mermaid-elk/elk.min.js"
     ]
 }
@@ -200,6 +209,7 @@ export default {
     WHEEL_ZOOM,
     FORCE_GRAPH,
     MERMAID,
+    MERMAID_ELK,
     EXCALIDRAW,
     MARKJS,
     I18NEXT,

src/public/app/services/mermaid.js

@@ -0,0 +1,28 @@
+import library_loader from "./library_loader.js";
+
+let elkLoaded = false;
+
+/**
+ * Determines whether the ELK extension of Mermaid.js needs to be loaded (which is a relatively large library), based on the
+ * front-matter of the diagram and loads the library if needed.
+ * 
+ * <p>
+ * If the library has already been loaded or the diagram does not require it, the method will exit immediately.
+ * 
+ * @param mermaidContent the plain text of the mermaid diagram, potentially including a frontmatter.
+ */
+export async function loadElkIfNeeded(mermaidContent) {
+    if (elkLoaded) {
+        // Exit immediately since the ELK library is already loaded.
+        return;
+    }
+
+    const parsedContent = await mermaid.parse(mermaidContent, {
+        suppressErrors: true 
+    });
+    if (parsedContent?.config?.layout === "elk") {
+        elkLoaded = true;
+        await library_loader.requireLibrary(library_loader.MERMAID_ELK);
+        mermaid.registerLayoutLoaders(MERMAID_ELK);
+    }    
+}

src/public/app/widgets/mermaid.js

@@ -3,6 +3,7 @@ import libraryLoader from "../services/library_loader.js";
 import NoteContextAwareWidget from "./note_context_aware_widget.js";
 import server from "../services/server.js";
 import utils from "../services/utils.js";
+import { loadElkIfNeeded } from "../services/mermaid.js";
 
 const TPL = `<div class="mermaid-widget">
     <style>
@@ -57,11 +58,10 @@ export default class MermaidWidget extends NoteContextAwareWidget {
         this.$errorContainer.hide();
 
         await libraryLoader.requireLibrary(libraryLoader.MERMAID);
-
+        
         const documentStyle = window.getComputedStyle(document.documentElement);
-        const mermaidTheme = documentStyle.getPropertyValue('--mermaid-theme');
-
-        mermaid.registerLayoutLoaders(MERMAID_ELK);
+        const mermaidTheme = documentStyle.getPropertyValue('--mermaid-theme');        
+        
         mermaid.mermaidAPI.initialize({
             startOnLoad: false,
             theme: mermaidTheme.trim(),
@@ -112,6 +112,7 @@ export default class MermaidWidget extends NoteContextAwareWidget {
                 zoomOnClick: false
             });
         } catch (e) {
+            console.warn(e);
             this.$errorMessage.text(e.message);
             this.$errorContainer.show();
         }
@@ -123,6 +124,7 @@ export default class MermaidWidget extends NoteContextAwareWidget {
         const blob = await this.note.getBlob();
         const content = blob.content || "";
 
+        await loadElkIfNeeded(content);
         const {svg} = await mermaid.mermaidAPI.render(`mermaid-graph-${idCounter}`, content);
         return svg;
     }