Answer stability round 3: the portable writer merges adjacent nested task lists after the split, so it never writes !adf:listBreak

src/markdown/portable/adf-to-portable-markdown.test.ts

@@ -134,6 +134,8 @@ test('spells a task list as a bullet list whose items lead with their state', ()
   assert.equal(portable(node('taskList', {}, blockTask, codeTask)), '- [x] First.\n\n  Second.\n- [ ]\n\n```\nx\n```\n')
   const parent = node('blockTaskItem', { state: 'TODO' }, said('q'), said('q2'))
   assert.equal(portable(node('taskList', {}, parent, node('taskList', {}, task('DONE', 'n')), task('DONE', 'B'))), '- [ ] q\n\n  q2\n  - [x] n\n- [x] B\n')
+  const holding = node('blockTaskItem', { state: 'TODO' }, said('A'), node('taskList', {}, task('TODO', 'c')))
+  assert.equal(portable(node('taskList', {}, holding, node('taskList', {}, task('TODO', 'd')), task('TODO', 'B'))), '- [ ] A\n  - [ ] c\n  - [ ] d\n- [ ] B\n')
   const listTask = node('blockTaskItem', { state: 'DONE' }, bulletList(item(said('a'))))
   assert.equal(portable(node('taskList', {}, task('TODO', ''), listTask, node('taskList', {}, task('TODO', 'b')))), '- \\[ ]\n- \\[x]\n- a\n- \\[ ] b\n')
   assert.equal(portable(node('taskList', {}, task('DONE', 'a'), node('taskList', {}, task('TODO', '')))), '- [x] a\n  - [ ]\n')

src/markdown/portable/adf-to-portable-markdown.ts

@@ -296,28 +296,14 @@ function reduceTaskList(node: AdfNode, reduction: Reduction): Result<AdfNode[]>
   }
   const regular = isTask(kept[0]?.child) && kept.every(({ child }) => isTask(child) || child.type === 'taskList')
   const tasks: AdfNode[] = []
-  let nested: AdfNode[] = []
   for (const { blocks, child } of kept) {
-    if (!regular) {
-      const standsAlone = !isTask(child) && child.type !== 'taskList'
-      for (const block of standsAlone ? [{ content: blocks, type: 'listItem' }] : blocks) tasks.push(block)
-      continue
-    }
-    if (isTask(child)) {
-      pushMerged(tasks, nested)
-      nested = []
-    }
-    for (const block of blocks) (isTask(child) ? tasks : nested).push(block)
+    const standsAlone = !regular && !isTask(child) && child.type !== 'taskList'
+    for (const block of standsAlone ? [{ content: blocks, type: 'listItem' }] : blocks) tasks.push(block)
   }
-  pushMerged(tasks, nested)
-  if (regular) return success(splitTaskList({ content: tasks, type: 'taskList' }))
+  if (regular) return success(splitTaskList({ content: tasks, type: 'taskList' }).map((block) => (block.type === 'taskList' ? { ...block, content: mergedLists(nodeContent(block)) } : block)))
   return success(listOf(nodeContent(tasksAsText({ content: tasks, type: 'taskList' })), 'bulletList'))
 }
 
-function pushMerged(tasks: AdfNode[], nested: readonly AdfNode[]): void {
-  for (const block of mergedLists(nested)) tasks.push(block)
-}
-
 function isTask(node: AdfNode | undefined): boolean {
   return node?.type === 'taskItem' || node?.type === 'blockTaskItem'
 }