Add loading spinners and mermaid error handling (#12358)

- Add loading spinners on editor and mermaid renderers
- Add error handling and inline error box for mermaid
- Fix Mermaid rendering by using the .init api

web_src/js/markdown/mermaid.js

@@ -1,23 +1,56 @@
-import {random} from '../utils.js';
+const MAX_SOURCE_CHARACTERS = 5000;
+
+function displayError(el, err) {
+  el.closest('pre').classList.remove('is-loading');
+  const errorNode = document.createElement('div');
+  errorNode.setAttribute('class', 'ui message error markdown-block-error mono');
+  errorNode.textContent = err.str || err.message || String(err);
+  el.closest('pre').before(errorNode);
+}
 
 export async function renderMermaid(els) {
   if (!els || !els.length) return;
 
-  const {mermaidAPI} = await import(/* webpackChunkName: "mermaid" */'mermaid');
+  const mermaid = await import(/* webpackChunkName: "mermaid" */'mermaid');
 
-  mermaidAPI.initialize({
-    startOnLoad: false,
+  mermaid.initialize({
+    mermaid: {
+      startOnLoad: false,
+    },
+    flowchart: {
+      useMaxWidth: true,
+      htmlLabels: false,
+    },
     theme: 'neutral',
     securityLevel: 'strict',
   });
 
   for (const el of els) {
-    mermaidAPI.render(`mermaid-${random(12)}`, el.textContent, (svg, bindFunctions) => {
-      const div = document.createElement('div');
-      div.classList.add('mermaid-chart');
-      div.innerHTML = svg;
-      if (typeof bindFunctions === 'function') bindFunctions(div);
-      el.closest('pre').replaceWith(div);
-    });
+    if (el.textContent.length > MAX_SOURCE_CHARACTERS) {
+      displayError(el, new Error(`Mermaid source of ${el.textContent.length} characters exceeds the maximum allowed length of ${MAX_SOURCE_CHARACTERS}.`));
+      continue;
+    }
+
+    let valid;
+    try {
+      valid = mermaid.parse(el.textContent);
+    } catch (err) {
+      displayError(el, err);
+    }
+
+    if (!valid) {
+      el.closest('pre').classList.remove('is-loading');
+      continue;
+    }
+
+    try {
+      mermaid.init(undefined, el, (id) => {
+        const svg = document.getElementById(id);
+        svg.classList.add('mermaid-chart');
+        svg.closest('pre').replaceWith(svg);
+      });
+    } catch (err) {
+      displayError(el, err);
+    }
   }
 }