android/app/src/main/java/com/fabledsword/thoughtsync/ui/EditorBlock.kt

@@ -167,11 +167,13 @@ fun List<EditorBlock>.plusTask(): Pair<List<EditorBlock>, Long> {
  * only the way it is drawn.
  */
 internal fun List<EditorBlock>.promotingTasks(index: Int): List<EditorBlock> {
-    val block = getOrNull(index) ?: return this
+    val block = getOrNull(index)
-    if (block.isTask) return this
+    if (block == null || block.isTask) return this
     val split = splitBlocks(block.value.text, nextId())
-    if (split.size == 1 && !split.first().isTask) return this
+    // A single prose block back means there was nothing to promote. `splitBlocks` never
-    return take(index) + split + drop(index + 1)
+    // returns an empty list, so `first()` is safe.
+    val changed = split.size > 1 || split.first().isTask
+    return if (changed) take(index) + split + drop(index + 1) else this
 }
 
 /**

frontend/src/notes/blocks.ts

@@ -137,6 +137,8 @@ export function promoteTasks(blocks: EditorBlock[], index: number): EditorBlock[
   const block = blocks[index];
   if (!block || block.checked !== null) return blocks;
   const split = splitBlocks(block.text, nextId(blocks));
-  if (split.length === 1 && split[0].checked === null) return blocks;
+  // A single prose block back means there was nothing to promote. `splitBlocks` never
-  return [...blocks.slice(0, index), ...split, ...blocks.slice(index + 1)];
+  // returns an empty array, so `split[0]` is safe.
+  const changed = split.length > 1 || split[0].checked !== null;
+  return changed ? [...blocks.slice(0, index), ...split, ...blocks.slice(index + 1)] : blocks;
 }