From ffe57ab0ec2b07467c5f511bb650bb75fa08d624 Mon Sep 17 00:00:00 2001
From: Nico de Haen <ndh@deltachat.de>
Date: Mon, 7 Sep 2026 22:00:49 +0200
Subject: [PATCH] fix: syncing failures

---
 src/App.tsx | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/App.tsx b/src/App.tsx
index d52c7c7..4d3e0f8 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -14,6 +14,10 @@ import "@ironcalc/workbook/style.css";
 import { Webxdc } from "@webxdc/types";
 import { encode as base64Encode, decode as base64Decode } from "base64-arraybuffer";
 
+// Concurrent WASM initialization can replace the instance backing existing
+// models. Share the pending initialization across StrictMode effect runs.
+let initialization: ReturnType<typeof init> | undefined;
+
 declare global {
   interface Window {
     webxdc: Webxdc<{ data: string, sender: string }>;
@@ -53,8 +57,8 @@ function App() {
     // sure we don't actually 
     let cancelled = false;
     async function start() {
-      // initialize ironcalc. This is idempotent so can be called multiple times in StrictMode.
-      await init();
+      initialization ??= init();
+      await initialization;
       // the first time around in StrictMode, we have been cancelled so
       // we don't attempt to set the model.
       if (cancelled) {
@@ -62,7 +66,15 @@ function App() {
       }
       // the second time in StrictMode (or the first time outside of StrictMode) is the real one
       // so we load the model from storage or create a new one and set it in state.
-      const newModel = loadSelectedModelFromStorage() ?? createNewModel();
+      const storedModel = loadSelectedModelFromStorage();
+      if (!storedModel) {
+        // A replay cursor is only valid together with the workbook it describes.
+        localStorage.removeItem("last_serial");
+        // With no cached local edits, historical updates from this device
+        // must also be replayed instead of being mistaken for live echoes.
+        localStorage.removeItem("uuid");
+      }
+      const newModel = storedModel ?? createNewModel();
       setModel(newModel);
     }
     // we start the async here, not waiting for its completion
@@ -107,19 +119,21 @@ function App() {
 
   // Incoming: apply remote diffs to the model.
   useEffect(() => {
-    if (listenerRegistered.current) {
+    // Register only after WASM and the workbook are ready.
+    // Delta Chat retains updates and replays them when we register.
+    if (!model || listenerRegistered.current) {
       return
     }
     listenerRegistered.current = true
     window.webxdc.setUpdateListener((update) => {
       const model = modelRef.current;
       const payload = update.payload;
-      localStorage.setItem("last_serial", update.serial.toString());
       if (!model) {
         console.warn("Received external diffs but model is not initialized yet");
         return
       }
       if (payload.sender === uuid) {
+        localStorage.setItem("last_serial", update.serial.toString());
         return
       }
       // Decode base64 back to binary and convert to Uint8Array
@@ -127,11 +141,13 @@ function App() {
       const diff = new Uint8Array(diffBuffer);
       model.applyExternalDiffs(diff);
       saveSelectedModelInStorage(model);
+      // Never acknowledge an update before its changes have been saved.
+      localStorage.setItem("last_serial", update.serial.toString());
       // The model is mutated in place; bump the revision so IronCalc repaints
       // the canvas without disturbing an in-progress edit.
       setExternalRevision((revision) => revision + 1);
     }, get_last_serial())
-  }, [uuid])
+  }, [model, uuid])
 
 
   if (!model) {