apps/client/src/features/page/tree/model/tree-model.test.ts

@@ -752,6 +752,27 @@ describe("treeModel.placeByPosition", () => {
     });
     expect(t.map((n) => n.id)).toEqual(["r1", "child", "r2", "rp"]);
   });
+
+  it("returns same reference (no-op) when the destination parent is inside the source's own subtree (#206 ui-state-races-1)", () => {
+    // Moving `a` under its own descendant `b` is a cycle. Without the guard,
+    // remove(a) drops b too and insertByPosition can't re-place a -> the whole
+    // subtree silently vanishes. The guard refuses the move (same reference).
+    const cyclic: P[] = [
+      {
+        id: "a",
+        name: "A",
+        position: "a0",
+        children: [{ id: "b", name: "B", position: "a1" }],
+      },
+    ];
+    const t = treeModel.placeByPosition(cyclic, "a", {
+      parentId: "b",
+      position: "a5",
+    });
+    expect(t).toBe(cyclic);
+    expect(treeModel.find(t, "a")).not.toBeNull();
+    expect(treeModel.find(t, "b")).not.toBeNull();
+  });
 });
 
 describe("treeModel.move", () => {

apps/client/src/features/page/tree/model/tree-model.ts

@@ -294,6 +294,20 @@ export const treeModel = {
     const source = treeModel.find(tree, sourceId);
     if (!source) return tree;
     if (to.parentId !== null && !treeModel.find(tree, to.parentId)) return tree;
+    // Cycle guard, mirroring `move`'s `isDescendant` check (#206 ui-state-races-1).
+    // If the destination parent is INSIDE the moved node's own subtree (reachable
+    // when server-authoritative move events arrive out of order — e.g. X moved
+    // under Y, then Y under X, but on this receiver Y is still inside X), then
+    // `remove(sourceId)` would drop the future parent along with the whole subtree
+    // and `insertByPosition` could not find it again — the node and ALL its
+    // descendants would silently vanish. Refuse the move and return the same
+    // reference so callers can detect the no-op and reconcile (refetch) instead.
+    if (
+      to.parentId !== null &&
+      treeModel.isDescendant(tree, sourceId, to.parentId)
+    ) {
+      return tree;
+    }
     const removed = treeModel.remove(tree, sourceId);
     // Reuse the same position-ordered insertion as `insertByPosition` by
     // stamping the authoritative position onto the moved node first.

apps/client/src/features/websocket/tree-socket-reducers.test.ts

@@ -183,6 +183,34 @@ describe("applyMoveTreeNode", () => {
     expect(moved?.hasChildren).toBe(true);
     expect(moved?.position).toBe("a4");
   });
+
+  it("does NOT drop a subtree on a cyclic/out-of-order move (parent inside source) (#206 ui-state-races-1)", () => {
+    // Locally `b` is still nested inside `a` (an earlier "a under b" echo hasn't
+    // applied yet). An out-of-order "move a under b" event now arrives — b is a
+    // descendant of a, so re-parenting would make placeByPosition remove a (and
+    // its whole subtree, incl. b) and fail to re-insert. Before the fix BOTH a
+    // and b silently vanished; now the reducer leaves the tree untouched.
+    const tree: SpaceTreeNode[] = [
+      node("a", {
+        position: "a0",
+        hasChildren: true,
+        children: [node("b", { position: "a1", parentPageId: "a" })],
+      }),
+    ];
+    const next = applyMoveTreeNode(tree, {
+      id: "a",
+      parentId: "b",
+      oldParentId: null,
+      index: 0,
+      position: "a4",
+      pageData: {},
+    });
+    // No silent data loss: both nodes survive.
+    expect(treeModel.find(next, "a")).not.toBeNull();
+    expect(treeModel.find(next, "b")).not.toBeNull();
+    // The cyclic move is refused as a no-op (same reference) pending reconcile.
+    expect(next).toBe(tree);
+  });
 });
 
 describe("applyDeleteTreeNode", () => {

apps/client/src/features/websocket/tree-socket-reducers.ts

@@ -76,6 +76,19 @@ export function applyMoveTreeNode(
   const oldParentId = (sourceBefore as SpaceTreeNode).parentPageId ?? null;
   const newParentId = payload.parentId as string | null;
 
+  // Cyclic / out-of-order move guard (#206 ui-state-races-1): if the
+  // authoritative new parent is currently INSIDE the moved node's own subtree on
+  // this client (e.g. server moved X under Y then Y under X and the events
+  // arrived such that Y is still nested in X here), re-parenting is impossible to
+  // represent locally. `placeByPosition` returns `prev` for this, but the
+  // `placed === prev` fallback below would then `remove` the source — dropping
+  // the node AND every descendant (incl. the would-be parent) silently. Leave the
+  // tree untouched instead; a later corrective event or a reconnect refetch
+  // reconciles it. Never delete a subtree we cannot safely re-place.
+  if (newParentId && treeModel.isDescendant(prev, payload.id, newParentId)) {
+    return prev;
+  }
+
   // Place the node by its fractional `position` among the new siblings — NOT by
   // the sender's absolute `index` (the sender computed that against its own
   // loaded set, which differs from this receiver's). Using the position keeps