fix undoing setAttribute combined with delete corrupts remote state - closes #757

When an attribute change and a delete on the same XmlText are bundled in
a single undo stack item, the redone attribute item kept an `origin`
pointing at an item under the now-deleted original parent. The encoder
then omitted parent info (origin was non-null), and the remote derived
parent from origin -- attaching the attribute to the wrong, deleted
type. The restored attribute went missing on sync.

In redoItem, after determining `left` for parentSub items, drop it if
it refers to a different parent than the redo target. This forces the
encoder to write parent info directly, so the remote attaches the
attribute to the correct (redone) parent.

src/structs/Item.js

@@ -223,6 +223,10 @@ export const redoItem = (transaction, item, redoitems, itemsToDelete, ignoreRemo
     } else {
       left = parentType._map.get(item.parentSub) || null
     }
+    // drop cross-parent left so origin doesn't mislead the remote (#757)
+    if (left !== null && left.parent !== parentType) {
+      left = null
+    }
   }
   const nextClock = getState(store, ownClientID)
   const nextId = createID(ownClientID, nextClock)

tests/undo-redo.tests.js

@@ -811,3 +811,35 @@ export const testUndoDoingStackItem = async (_tc) => {
   t.compare(metaRedo, '42', 'currStackItem is accessible while redoing')
   t.compare(undoManager.currStackItem, null, 'currStackItem is null after observe/transaction')
 }
+
+/**
+ * @see https://github.com/yjs/yjs/issues/757
+ * @param {t.TestCase} _tc
+ */
+export const testUndoSetAttributeAndDeleteSyncsAttributes = _tc => {
+  const doc = new Y.Doc()
+  const root = /** @type {Y.XmlText} */ (doc.get('sharedRoot', Y.XmlText))
+  const button = new Y.XmlText()
+  button.setAttribute('type', 'button')
+  button.setAttribute('test', true)
+  button.insert(0, 'Click me')
+  root.insertEmbed(0, button)
+
+  const undoManager = new Y.UndoManager(root)
+  undoManager.stopCapturing()
+
+  button.setAttribute('type', 'paragraph')
+  root.delete(0, 1)
+  undoManager.undo()
+
+  const expectedAttrs = { type: 'button', test: true }
+  const expectedText = 'Click me'
+  t.compare(root.toDelta()[0].insert.getAttributes(), expectedAttrs)
+  t.compare(root.toDelta()[0].insert.toString(), expectedText)
+
+  const remoteDoc = new Y.Doc()
+  Y.applyUpdateV2(remoteDoc, Y.encodeStateAsUpdateV2(doc))
+  const remoteRoot = /** @type {Y.XmlText} */ (remoteDoc.get('sharedRoot', Y.XmlText))
+  t.compare(remoteRoot.toDelta()[0].insert.getAttributes(), expectedAttrs)
+  t.compare(remoteRoot.toDelta()[0].insert.toString(), expectedText)
+}