diff --git a/build/splash.bmp b/build/splash.bmp
new file mode 100644
index 0000000..ecc9a40
Binary files /dev/null and b/build/splash.bmp differ
diff --git a/docs/release-notes/v1.1.1.md b/docs/release-notes/v1.1.1.md
new file mode 100644
index 0000000..a5aa4d1
--- /dev/null
+++ b/docs/release-notes/v1.1.1.md
@@ -0,0 +1,16 @@
+## Fixes the Windows app not opening
+
+### Fixed
+- **The app could fail to open on Windows, and then do nothing on every later launch.** If Windows reserved the app's local port (common with Hyper-V, WSL, or Docker), v1.1.0 never showed a window and kept running in the background. It now falls back to another port, and if it truly can't start it shows an error instead of hanging.
+- **The engine didn't start in the desktop app.** The multi-threaded engine needs a browser feature that the desktop shell wasn't enabling. It's now turned on explicitly, so the full-strength, multi-core engine runs as intended.
+- Launching the app while it's already open now brings up the existing window, and opens a new one if there isn't one.
+
+### Improved
+- The Windows portable `.exe` now shows a splash screen while it unpacks, so you can tell it's starting.
+
+### If v1.1.0 wouldn't open for you
+A hidden v1.1.0 process may still be running. Open Task Manager, end any **Chess Solver** processes (or run `taskkill /IM "Chess Solver.exe" /F`), then launch v1.1.1.
+
+### Downloads
+- **Windows:** `Chess.Solver.1.1.1.exe`, a portable build. SmartScreen may warn because it's unsigned: choose *More info → Run anyway*.
+- **macOS:** `Chess.Solver-1.1.1-universal.dmg` (Intel + Apple Silicon). It isn't notarized, so right-click the app → **Open** on first launch.
diff --git a/electron/main.cjs b/electron/main.cjs
index 6b4402d..805f54e 100644
--- a/electron/main.cjs
+++ b/electron/main.cjs
@@ -1,4 +1,4 @@
-const { app, BrowserWindow, shell } = require('electron');
+const { app, BrowserWindow, dialog, shell } = require('electron');
 const path = require('path');
 const http = require('http');
 const fs = require('fs');
@@ -20,7 +20,9 @@ const MIME_TYPES = {
 // A fixed port keeps the app's origin stable between launches. localStorage
 // (theme, line count, think time) and the HTTP cache are keyed by origin, so a
 // random port silently reset settings every launch and forced a full
-// recompile of the 100MB+ engine. Falls back to any free port if taken.
+// recompile of the 100MB+ engine. Falls back to any free port if the
+// preferred one can't be used — taken (EADDRINUSE) or, on Windows, inside a
+// range reserved by Hyper-V/WSL/Docker (EACCES).
 const PREFERRED_PORT = 47813;
 
 // 'wasm-unsafe-eval' and blob: workers are required by the multi-threaded
@@ -108,8 +110,11 @@ function startServer() {
     const server = http.createServer(handleRequest);
     const listen = (port) => server.listen(port, '127.0.0.1');
     server.once('listening', () => resolve(server.address().port));
+    let triedFallback = false;
     server.on('error', (err) => {
-      if (err.code === 'EADDRINUSE' && server.address() === null) {
+      if (!triedFallback && server.address() === null) {
+        triedFallback = true;
+        console.warn(`[server] port ${PREFERRED_PORT} unavailable (${err.code}); using a random port`);
         listen(0);
       } else {
         reject(err);
@@ -119,6 +124,13 @@ function startServer() {
   });
 }
 
+// The multi-threaded Stockfish build needs SharedArrayBuffer. The server's
+// COOP/COEP headers make browsers cross-origin isolate the page, but
+// Electron's BrowserWindow doesn't reliably honor them (crossOriginIsolated
+// stays false), which left SharedArrayBuffer undefined and the engine unable
+// to start. Enable it explicitly; must run before the app is ready.
+app.commandLine.appendSwitch('enable-features', 'SharedArrayBuffer');
+
 let appOrigin = null;
 
 function createWindow() {
@@ -170,16 +182,29 @@ if (!gotInstanceLock) {
     const [win] = BrowserWindow.getAllWindows();
     if (win) {
       if (win.isMinimized()) win.restore();
+      win.show();
       win.focus();
+    } else if (appOrigin !== null) {
+      createWindow();
     }
   });
 }
 
 app.whenReady().then(async () => {
   if (!gotInstanceLock) return;
-  const port = await startServer();
-  appOrigin = `http://127.0.0.1:${port}`;
-  createWindow();
+  try {
+    const port = await startServer();
+    appOrigin = `http://127.0.0.1:${port}`;
+    createWindow();
+  } catch (err) {
+    // Never linger as a windowless background process: it would hold the
+    // single-instance lock and make every later launch silently do nothing.
+    dialog.showErrorBox(
+      'Chess Solver could not start',
+      `The app's local server failed to start:\n\n${err && err.message ? err.message : err}`
+    );
+    app.exit(1);
+  }
 });
 
 // macOS convention: the app stays alive with no windows and reopens one when
diff --git a/package-lock.json b/package-lock.json
index b7ebf6b..c12a63d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "chess-solver",
-  "version": "1.1.0",
+  "version": "1.1.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "chess-solver",
-      "version": "1.1.0",
+      "version": "1.1.1",
       "license": "MIT",
       "dependencies": {
         "@fontsource/dm-sans": "^5.3.0",
diff --git a/package.json b/package.json
index e5e8252..b5c9d2f 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "chess-solver",
   "private": true,
-  "version": "1.1.0",
+  "version": "1.1.1",
   "description": "Desktop chess analysis tool powered by Stockfish 18 — set up any position and get engine-recommended moves with evaluations, tactical motifs, and principal variations.",
   "author": "ccharafeddine",
   "license": "MIT",
@@ -37,6 +37,9 @@
       "target": "portable",
       "icon": "build/icon.ico"
     },
+    "portable": {
+      "splashImage": "build/splash.bmp"
+    },
     "mac": {
       "target": [
         {
diff --git a/progress.txt b/progress.txt
index 731f441..e185e48 100644
--- a/progress.txt
+++ b/progress.txt
@@ -1,5 +1,20 @@
 # Chess Solver — Progress
 
+## v1.1.1 (2026-09-24): Windows launch fix
+- v1.1.0 .exe did nothing on launch. Two causes: (1) a listen error other
+  than EADDRINUSE on the fixed port (Windows returns EACCES for ports in
+  Hyper-V/WSL/Docker excluded ranges) rejected startServer unhandled, so no
+  window was created but the process lived on, holding the single-instance
+  lock and making every later launch exit silently. Now any listen error
+  falls back to a random port, and a total failure shows an error box and
+  exits. (2) Electron's BrowserWindow doesn't honor COOP/COEP
+  (crossOriginIsolated stays false), so SharedArrayBuffer was undefined and
+  the multi-threaded engine never started; fixed with
+  app.commandLine.appendSwitch('enable-features', 'SharedArrayBuffer').
+  Both verified in real Electron under Xvfb (engine runs 3 threads, ~1M
+  nps; port-taken fallback; second-instance handoff).
+- Portable .exe shows build/splash.bmp while it unpacks (~320MB per launch).
+
 ## v1.1.0 (2026-09-24): engine speed/strength + reliability audit
 - The app had been shipping the SINGLE-threaded Stockfish build
   (public/stockfish.* byte-matched stockfish-18-single), so the Threads