Make task list checkboxes clickable in the preview tab (#37010)

When a checkbox is toggled in the markup preview tab, the change is now
synced back to the editor textarea. Extracted a `toggleTasklistCheckbox`
helper to deduplicate the byte-offset toggle logic.

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

web_src/js/features/comp/ComboMarkdownEditor.ts

@@ -10,6 +10,7 @@ import {
 } from './EditorUpload.ts';
 import {handleGlobalEnterQuickSubmit} from './QuickSubmit.ts';
 import {renderPreviewPanelContent} from '../repo-editor.ts';
+import {toggleTasklistCheckbox} from '../../markup/tasklist.ts';
 import {easyMDEToolbarActions} from './EasyMDEToolbarActions.ts';
 import {initTextExpander} from './TextExpander.ts';
 import {showErrorToast} from '../../modules/toast.ts';
@@ -236,6 +237,20 @@ export class ComboMarkdownEditor {
       const response = await POST(this.previewUrl, {data: formData});
       const data = await response.text();
       renderPreviewPanelContent(panelPreviewer, data);
+      // enable task list checkboxes in preview and sync state back to the editor
+      for (const checkbox of panelPreviewer.querySelectorAll<HTMLInputElement>('.task-list-item input[type=checkbox]')) {
+        checkbox.disabled = false;
+        checkbox.addEventListener('input', () => {
+          const position = parseInt(checkbox.getAttribute('data-source-position')!) + 1;
+          const newContent = toggleTasklistCheckbox(this.value(), position, checkbox.checked);
+          if (newContent === null) {
+            checkbox.checked = !checkbox.checked;
+            return;
+          }
+          this.value(newContent);
+          triggerEditorContentChanged(this.container);
+        });
+      }
     });
   }
 

web_src/js/markup/tasklist.test.ts

@@ -0,0 +1,9 @@
+import {toggleTasklistCheckbox} from './tasklist.ts';
+
+test('toggleTasklistCheckbox', () => {
+  expect(toggleTasklistCheckbox('- [ ] task', 3, true)).toEqual('- [x] task');
+  expect(toggleTasklistCheckbox('- [x] task', 3, false)).toEqual('- [ ] task');
+  expect(toggleTasklistCheckbox('- [ ] task', 0, true)).toBeNull();
+  expect(toggleTasklistCheckbox('- [ ] task', 99, true)).toBeNull();
+  expect(toggleTasklistCheckbox('😀 - [ ] task', 8, true)).toEqual('😀 - [x] task');
+});

web_src/js/markup/tasklist.ts

@@ -3,6 +3,23 @@ import {showErrorToast} from '../modules/toast.ts';
 
 const preventListener = (e: Event) => e.preventDefault();
 
+/**
+ * Toggle a task list checkbox in markdown content.
+ * `position` is the byte offset of the space or `x` character inside `[ ]`.
+ * Returns the updated content, or null if the position is invalid.
+ */
+export function toggleTasklistCheckbox(content: string, position: number, checked: boolean): string | null {
+  const buffer = new TextEncoder().encode(content);
+  // Indexes may fall off the ends and return undefined.
+  if (buffer[position - 1] !== '['.charCodeAt(0) ||
+    buffer[position] !== ' '.charCodeAt(0) && buffer[position] !== 'x'.charCodeAt(0) ||
+    buffer[position + 1] !== ']'.charCodeAt(0)) {
+    return null;
+  }
+  buffer[position] = checked ? 'x'.charCodeAt(0) : ' '.charCodeAt(0);
+  return new TextDecoder().decode(buffer);
+}
+
 /**
  * Attaches `input` handlers to markdown rendered tasklist checkboxes in comments.
  *
@@ -23,24 +40,17 @@ export function initMarkupTasklist(elMarkup: HTMLElement): void {
 
     checkbox.setAttribute('data-editable', 'true');
     checkbox.addEventListener('input', async () => {
-      const checkboxCharacter = checkbox.checked ? 'x' : ' ';
       const position = parseInt(checkbox.getAttribute('data-source-position')!) + 1;
 
       const rawContent = container.querySelector('.raw-content')!;
       const oldContent = rawContent.textContent;
 
-      const encoder = new TextEncoder();
+      const newContent = toggleTasklistCheckbox(oldContent, position, checkbox.checked);
-      const buffer = encoder.encode(oldContent);
+      if (newContent === null) {
-      // Indexes may fall off the ends and return undefined.
+        // Position is probably wrong. Revert and don't allow change.
-      if (buffer[position - 1] !== '['.codePointAt(0) ||
-        buffer[position] !== ' '.codePointAt(0) && buffer[position] !== 'x'.codePointAt(0) ||
-        buffer[position + 1] !== ']'.codePointAt(0)) {
-        // Position is probably wrong.  Revert and don't allow change.
         checkbox.checked = !checkbox.checked;
         throw new Error(`Expected position to be space or x and surrounded by brackets, but it's not: position=${position}`);
       }
-      buffer.set(encoder.encode(checkboxCharacter), position);
-      const newContent = new TextDecoder().decode(buffer);
 
       if (newContent === oldContent) {
         return;