fix: Dropped content in Markdown parser with mixed checklist content (#11994)

* fix: Dropped content in Markdown parser with mixed checklist content

* fix: Treat non-checkbox items as unchecked in mixed checkbox lists

When a bullet list contains a mix of checkbox and regular items, the
markdown-it checkbox rule converts the list to a checkbox_list but
leaves non-checkbox items as list_item tokens. Since the Prosemirror
schema requires checkbox_item+ children, these invalid list_item nodes
cause the entire list to be silently dropped — explaining the content
truncation reported in #11988.

Convert remaining list_item tokens that are direct children of a
checkbox_list into unchecked checkbox_item tokens. Uses a level stack
to avoid converting nested bullet/ordered list items.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: Move checkbox tests to collocated checkboxes.test.ts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

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

server/editor/checkboxes.test.ts

@@ -0,0 +1,67 @@
+import { parser, serializer } from "@server/editor";
+
+interface ProsemirrorNode {
+  type: string;
+  content?: ProsemirrorNode[];
+  attrs?: Record<string, unknown>;
+}
+
+it("preserves mixed checkbox and regular items in a list", () => {
+  const markdown = `- [x] Checked item
+- Regular item
+- [ ] Unchecked item`;
+
+  const ast = parser.parse(markdown);
+  const json = ast?.toJSON();
+
+  const checkboxList = json?.content?.find(
+    (node: ProsemirrorNode) => node.type === "checkbox_list"
+  );
+
+  expect(checkboxList).toBeDefined();
+  expect(checkboxList?.content).toHaveLength(3);
+  expect(checkboxList?.content[0].type).toBe("checkbox_item");
+  expect(checkboxList?.content[1].type).toBe("checkbox_item");
+  expect(checkboxList?.content[2].type).toBe("checkbox_item");
+});
+
+it("round-trips mixed checkbox lists through serializer", () => {
+  const markdown = `- [x] Checked
+- Plain text
+- [ ] Unchecked`;
+
+  const ast = parser.parse(markdown);
+  const output = serializer.serialize(ast);
+
+  // All items should survive the round-trip
+  expect(output).toContain("Checked");
+  expect(output).toContain("Plain text");
+  expect(output).toContain("Unchecked");
+});
+
+it("does not convert nested bullet list items inside checkbox lists", () => {
+  const markdown = `- [x] Parent checkbox
+    - Nested bullet item
+    - Another nested item
+- [ ] Second checkbox`;
+
+  const ast = parser.parse(markdown);
+  const json = ast?.toJSON();
+
+  const checkboxList = json?.content?.find(
+    (node: ProsemirrorNode) => node.type === "checkbox_list"
+  );
+
+  expect(checkboxList).toBeDefined();
+  expect(checkboxList?.content).toHaveLength(2);
+  expect(checkboxList?.content[0].type).toBe("checkbox_item");
+  expect(checkboxList?.content[1].type).toBe("checkbox_item");
+
+  // Nested list should remain a bullet_list, not a checkbox_list
+  const nestedContent = checkboxList?.content[0].content;
+  const nestedList = nestedContent?.find(
+    (node: ProsemirrorNode) => node.type === "bullet_list"
+  );
+  expect(nestedList).toBeDefined();
+  expect(nestedList?.content?.[0].type).toBe("list_item");
+});

server/editor/index.test.ts

@@ -18,11 +18,21 @@ test("parses lowercase alpha lists", () => {
         attrs: { listStyle: "lower-alpha", order: 1 },
         content: [
           {
-            content: [{ content: [{ text: "First item", type: "text" }], type: "paragraph" }],
+            content: [
+              {
+                content: [{ text: "First item", type: "text" }],
+                type: "paragraph",
+              },
+            ],
             type: "list_item",
           },
           {
-            content: [{ content: [{ text: "Second item", type: "text" }], type: "paragraph" }],
+            content: [
+              {
+                content: [{ text: "Second item", type: "text" }],
+                type: "paragraph",
+              },
+            ],
             type: "list_item",
           },
         ],
@@ -42,11 +52,21 @@ test("parses uppercase alpha lists", () => {
         attrs: { listStyle: "upper-alpha", order: 1 },
         content: [
           {
-            content: [{ content: [{ text: "First item", type: "text" }], type: "paragraph" }],
+            content: [
+              {
+                content: [{ text: "First item", type: "text" }],
+                type: "paragraph",
+              },
+            ],
             type: "list_item",
           },
           {
-            content: [{ content: [{ text: "Second item", type: "text" }], type: "paragraph" }],
+            content: [
+              {
+                content: [{ text: "Second item", type: "text" }],
+                type: "paragraph",
+              },
+            ],
             type: "list_item",
           },
         ],
@@ -68,7 +88,9 @@ b. Do that.`;
   const json = ast?.toJSON();
 
   // Find the ordered_list in the result
-  const orderedList = json?.content?.find((node: any) => node.type === "ordered_list");
+  const orderedList = json?.content?.find(
+    (node: { type: string }) => node.type === "ordered_list"
+  );
 
   expect(orderedList).toBeDefined();
   expect(orderedList?.attrs.listStyle).toBe("lower-alpha");
@@ -85,11 +107,21 @@ test("preserves numeric lists", () => {
         attrs: { listStyle: "number", order: 1 },
         content: [
           {
-            content: [{ content: [{ text: "First item", type: "text" }], type: "paragraph" }],
+            content: [
+              {
+                content: [{ text: "First item", type: "text" }],
+                type: "paragraph",
+              },
+            ],
             type: "list_item",
           },
           {
-            content: [{ content: [{ text: "Second item", type: "text" }], type: "paragraph" }],
+            content: [
+              {
+                content: [{ text: "Second item", type: "text" }],
+                type: "paragraph",
+              },
+            ],
             type: "list_item",
           },
         ],

shared/editor/rules/checkboxes.ts

@@ -106,6 +106,29 @@ export default function markdownItCheckbox(md: MarkdownIt): void {
       }
     }
 
+    // Second pass: convert any remaining direct child list_item tokens inside
+    // a checkbox_list to checkbox_item so they aren't silently dropped by the
+    // Prosemirror schema which requires checkbox_item+ children.
+    const checkboxListOpenLevels: number[] = [];
+    for (let i = 0; i < tokens.length; i++) {
+      const token = tokens[i];
+      if (token.type === "checkbox_list_open") {
+        checkboxListOpenLevels.push(token.level);
+      } else if (token.type === "checkbox_list_close") {
+        checkboxListOpenLevels.pop();
+      } else if (checkboxListOpenLevels.length > 0) {
+        const checkboxListOpenLevel =
+          checkboxListOpenLevels[checkboxListOpenLevels.length - 1];
+        const isDirectChild = token.level === checkboxListOpenLevel + 1;
+
+        if (isDirectChild && token.type === "list_item_open") {
+          token.type = "checkbox_item_open";
+        } else if (isDirectChild && token.type === "list_item_close") {
+          token.type = "checkbox_item_close";
+        }
+      }
+    }
+
     return false;
   });
 }