From 83092edc1cfcf329ec92bbe05e41f1e184ede713 Mon Sep 17 00:00:00 2001
From: Duane Johnson <duane@schoolai.com>
Date: Thu, 22 Jan 2026 17:12:39 -0700
Subject: [PATCH] fix: keep forked result and toJSON in sync during checkout

---
 .../src/encoding/shallow_snapshot.rs          |  2 +-
 crates/loro-wasm/tests/checkout.test.ts       | 38 +++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/crates/loro-internal/src/encoding/shallow_snapshot.rs b/crates/loro-internal/src/encoding/shallow_snapshot.rs
index bb9a6cf52..fb2f44c0e 100644
--- a/crates/loro-internal/src/encoding/shallow_snapshot.rs
+++ b/crates/loro-internal/src/encoding/shallow_snapshot.rs
@@ -254,7 +254,7 @@ pub(crate) fn encode_snapshot_at<W: std::io::Write>(
     w: &mut W,
 ) -> Result<(), LoroEncodeError> {
     let was_detached = doc.is_detached();
-    let version_before_start = doc.oplog_frontiers();
+    let version_before_start = doc.state_frontiers();
     doc._checkout_without_emitting(frontiers, true, false)
         .unwrap();
     let result = 'block: {
diff --git a/crates/loro-wasm/tests/checkout.test.ts b/crates/loro-wasm/tests/checkout.test.ts
index 78d2a98af..7dc9cd59f 100644
--- a/crates/loro-wasm/tests/checkout.test.ts
+++ b/crates/loro-wasm/tests/checkout.test.ts
@@ -76,4 +76,42 @@ describe("Checkout", () => {
     doc.import(docB.export({ mode: "update" }));
     expect(docB.cmpWithFrontiers(doc.frontiers())).toBe(0);
   });
+
+  it("forkAt inside subscription during checkout event should not corrupt state", async () => {
+    const doc = new LoroDoc();
+    doc.setPeerId("1");
+
+    // Make some changes
+    doc.getText("text").insert(0, "Hello");
+    doc.commit();
+    const frontier1 = doc.frontiers(); // [{ peer: "1", counter: 4 }]
+
+    doc.getText("text").insert(5, " World");
+    doc.commit();
+
+    // Verify initial state
+    expect(doc.toJSON()).toStrictEqual({ text: "Hello World" });
+
+    // Subscribe and call forkAt inside the callback
+    let forkResult: any = null;
+    doc.subscribe((event) => {
+      if (event.by === "checkout") {
+        // BUG: This corrupts the checkout state
+        const fork = doc.forkAt(doc.frontiers());
+        forkResult = fork.toJSON();
+      }
+    });
+
+    // Checkout to earlier state
+    doc.checkout(frontier1);
+
+    // Wait for events to be processed
+    await new Promise((resolve) => setTimeout(resolve, 1));
+
+    expect(doc.frontiers()).toStrictEqual(frontier1);
+    expect(doc.toJSON()).toStrictEqual({ text: "Hello" });
+
+    // The fork should also have the correct state at frontier1
+    expect(forkResult).toStrictEqual({ text: "Hello" });
+  });
 });