Add support for task list checkboxes outside p

Closes GH-80.
Closes GH-81.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: Mr0grog

lib/handlers/li.js

@@ -17,43 +17,17 @@ import {phrasing} from 'hast-util-phrasing'
  *   mdast node.
  */
 export function li(state, node) {
-  const head = node.children[0]
-  /** @type {boolean | null} */
-  let checked = null
-  /** @type {Element | undefined} */
-  let clone
-
-  // Check if this node starts with a checkbox.
-  if (head && head.type === 'element' && head.tagName === 'p') {
-    const checkbox = head.children[0]
-
-    if (
-      checkbox &&
-      checkbox.type === 'element' &&
-      checkbox.tagName === 'input' &&
-      checkbox.properties &&
-      (checkbox.properties.type === 'checkbox' ||
-        checkbox.properties.type === 'radio')
-    ) {
-      checked = Boolean(checkbox.properties.checked)
-      clone = {
-        ...node,
-        children: [
-          {...head, children: head.children.slice(1)},
-          ...node.children.slice(1)
-        ]
-      }
-    }
-  }
+  // If the list item starts with a checkbox, remove the checkbox and mark the
+  // list item as a GFM task list item.
+  const {cleanNode, checkbox} = extractLeadingCheckbox(node)
+  const checked = checkbox && Boolean(checkbox.properties.checked)
 
-  if (!clone) clone = node
-
-  const spread = spreadout(clone)
-  const children = state.toFlow(state.all(clone))
+  const spread = spreadout(cleanNode)
+  const children = state.toFlow(state.all(cleanNode))
 
   /** @type {ListItem} */
   const result = {type: 'listItem', spread, checked, children}
-  state.patch(clone, result)
+  state.patch(cleanNode, result)
   return result
 }
 
@@ -99,3 +73,61 @@ function spreadout(node) {
 
   return false
 }
+
+/**
+ * If the first bit of content in an element is a checkbox, create a copy of
+ * the element that does not include the checkbox and return the cleaned up
+ * copy alongside the checkbox that was removed. If there was no leading
+ * checkbox, this returns the original element unaltered (not a copy).
+ *
+ * This detects trees like:
+ *   `<li><input type="checkbox">Text</li>`
+ * And returns a tree like:
+ *   `<li>Text</li>`
+ *
+ * Or with nesting:
+ *   `<li><p><input type="checkbox">Text</p></li>`
+ * Which returns a tree like:
+ *   `<li><p>Text</p></li>`
+ *
+ * @param {Readonly<Element>} node
+ * @returns {{cleanNode: Element, checkbox: Element | null}}
+ */
+function extractLeadingCheckbox(node) {
+  const head = node.children[0]
+
+  if (
+    head &&
+    head.type === 'element' &&
+    head.tagName === 'input' &&
+    head.properties &&
+    (head.properties.type === 'checkbox' || head.properties.type === 'radio')
+  ) {
+    return {
+      cleanNode: {...node, children: node.children.slice(1)},
+      checkbox: head
+    }
+  }
+
+  // The checkbox may be nested in another element. If the first element has
+  // children, look for a leading checkbox inside it.
+  //
+  // NOTE: this only handles nesting in `<p>` elements, which is most common.
+  // It's possible a leading checkbox might be nested in other types of flow or
+  // phrasing elements (and *deeply* nested, which is not possible with `<p>`).
+  // Limiting things to `<p>` elements keeps this simpler for now.
+  if (head && head.type === 'element' && head.tagName === 'p') {
+    const {cleanNode: cleanHead, checkbox} = extractLeadingCheckbox(head)
+    if (checkbox) {
+      return {
+        cleanNode: {
+          ...node,
+          children: [cleanHead, ...node.children.slice(1)]
+        },
+        checkbox
+      }
+    }
+  }
+
+  return {cleanNode: node, checkbox: null}
+}

test/fixtures/ol/index.html

@@ -59,4 +59,14 @@
   <li><p><input type="checkbox">Echo</p></li>
   <li><p><input type="checkbox"><strong>Foxtrot</strong></p></li>
   <li><p><input type="checkbox"> <strong>Golf</strong></p></li>
+  <li>
+    <p>
+      <input type="checkbox"> Hotel
+    </p>
+  </li>
+  <li><input type="checkbox"> India</li>
+  <li>
+    <input type="checkbox"> Juliet
+  </li>
+  <li><input type="checkbox"></li>
 </ol>

test/fixtures/ol/index.md

@@ -57,3 +57,11 @@ Quuux.
 5. [ ] **Foxtrot**
 
 6. [ ] **Golf**
+
+7. [ ] Hotel
+
+8. [ ] India
+
+9. [ ] Juliet
+
+10.

test/fixtures/ul/index.html

@@ -59,4 +59,14 @@
   <li><p><input type="checkbox">Echo</p></li>
   <li><p><input type="checkbox"><strong>Foxtrot</strong></p></li>
   <li><p><input type="checkbox"> <strong>Golf</strong></p></li>
+  <li>
+    <p>
+      <input type="checkbox"> Hotel
+    </p>
+  </li>
+  <li><input type="checkbox"> India</li>
+  <li>
+    <input type="checkbox"> Juliet
+  </li>
+  <li><input type="checkbox"></li>
 </ul>

test/fixtures/ul/index.md

@@ -57,3 +57,11 @@ Quuux.
 * [ ] **Foxtrot**
 
 * [ ] **Golf**
+
+* [ ] Hotel
+
+* [ ] India
+
+* [ ] Juliet
+
+*