Fix counter undo after remote updates

crates/loro-internal/src/event.rs

@@ -431,12 +431,8 @@ impl Diff {
             (Diff::Map(a), Diff::Map(b)) => a.transform(b, left_prior),
             (Diff::Tree(a), Diff::Tree(b)) => a.transform(b, left_prior),
             #[cfg(feature = "counter")]
-            (Diff::Counter(a), Diff::Counter(b)) => {
-                if left_prior {
-                    *a += b;
-                } else {
-                    *a -= b;
-                }
+            (Diff::Counter(_a), Diff::Counter(_b)) => {
+                // Counter operations commute; no transformation is needed.
             }
             _ => {}
         }

crates/loro/tests/issue.rs

@@ -220,6 +220,40 @@ fn test_event_hint_merge_bug_clear_demonstration() {
     assert_eq!(text_b.to_string(), "bcde");
 }
 
+#[test]
+fn test_undo_counter_after_remote_update_issue_905() {
+    let doc_a = LoroDoc::new();
+    doc_a.set_peer_id(1).unwrap();
+    let mut undo_manager = UndoManager::new(&doc_a);
+    undo_manager.set_merge_interval(0);
+
+    let counter_a = doc_a.get_counter("counter");
+    counter_a.increment(1.0).unwrap();
+    doc_a.commit();
+
+    let doc_b = LoroDoc::new();
+    doc_b.set_peer_id(2).unwrap();
+    doc_b.import(&doc_a.export(ExportMode::all_updates()).unwrap())
+        .unwrap();
+
+    let counter_b = doc_b.get_counter("counter");
+    assert_eq!(counter_b.get_value(), 1.0);
+    counter_b.increment(1.0).unwrap();
+    doc_b.commit();
+
+    doc_a.import(&doc_b.export(ExportMode::all_updates()).unwrap())
+        .unwrap();
+    assert_eq!(counter_a.get_value(), 2.0);
+
+    assert!(undo_manager.can_undo());
+    assert!(undo_manager.undo().unwrap());
+    assert_eq!(counter_a.get_value(), 1.0);
+
+    assert!(undo_manager.can_redo());
+    assert!(undo_manager.redo().unwrap());
+    assert_eq!(counter_a.get_value(), 2.0);
+}
+
 #[test]
 fn import_twice() {
     let doc = LoroDoc::new();