fix: session switch race — mount terminal AFTER /connect POST completes

_openTerminal(name) was inside a 260ms setTimeout that fired before the
/connect POST completed. The WebSocket connected to the OLD ttyd (still
serving the previous session). Fix: animation runs concurrently with
/connect as before, but _openTerminal only fires after BOTH resolve.

Sequence: start animation + POST concurrently → await POST → await
animation → mount terminal. The new ttyd is guaranteed to be serving
the correct session before the WebSocket connects.

Added timerId null-check so animDone resolves immediately in test envs
where setTimeout is stubbed, preventing test hangs.

muxplex/frontend/app.js

@@ -657,21 +657,24 @@ async function openSession(name, opts = {}) {
     tile.style.height = '100vh';
   }
 
-  // After animation completes: switch views, then mount terminal
-  setTimeout(() => {
-    const overview = $('view-overview');
-    const expanded = $('view-expanded');
+  // Start animation concurrently with /connect POST — resolve when view is ready
+  var animDone = new Promise(function (resolve) {
+    var timerId = setTimeout(function () {
+      var overview = $('view-overview');
+      var expanded = $('view-expanded');
       if (overview) overview.style.display = 'none';
       if (expanded) {
         expanded.classList.remove('hidden');     // must remove class — !important wins over style.display
         expanded.classList.add('view--active');  // makes it display:flex
       }
-    // Mount terminal AFTER view is visible so FitAddon measures real dimensions
-    if (window._openTerminal) window._openTerminal(name);
       // Re-render sidebar after DOM is visible and dimensions are correct
       initSidebar();
       renderSidebar(_currentSessions, name);
+      resolve();
     }, 260);
+    // If setTimeout is stubbed (e.g. in test env), resolve immediately so we don't hang
+    if (timerId == null) resolve();
+  });
 
   // Mobile pill
   if (isMobile()) {
@@ -685,7 +688,7 @@ async function openSession(name, opts = {}) {
     updateSessionPill(_currentSessions);
   }
 
-  // Connect to session (before animation completes so ttyd is ready)
+  // Connect to session (kill old ttyd, spawn new one for this session)
   try {
     if (!opts.skipConnect) {
       await api('POST', `/api/sessions/${name}/connect`);
@@ -694,6 +697,12 @@ async function openSession(name, opts = {}) {
     showToast(err.message || 'Connection failed');
     return closeSession();
   }
+
+  // Wait for animation to finish (may already be done if /connect was slow)
+  await animDone;
+
+  // Mount terminal NOW — /connect has completed, new ttyd is serving the correct session
+  if (window._openTerminal) window._openTerminal(name);
 }
 
 /**

muxplex/frontend/tests/test_app.mjs

@@ -45,6 +45,7 @@ import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { fileURLToPath } from 'node:url';
 import { dirname, join } from 'node:path';
+import fs from 'node:fs';
 
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = dirname(__filename);
@@ -2025,4 +2026,28 @@ test('bindSidebarClickAway registers click listener on terminal-container', () =
   globalThis.document.getElementById = origGetById;
 });
 
+test('openSession mounts terminal AFTER connect POST, not inside animation timer', () => {
+  const source = fs.readFileSync(
+    new URL('../app.js', import.meta.url), 'utf8'
+  );
+
+  // Find the openSession function body
+  const fnStart = source.indexOf('async function openSession');
+  const fnBody = source.substring(fnStart, fnStart + 3000);
+
+  // _openTerminal must NOT appear inside setTimeout
+  const setTimeoutIdx = fnBody.indexOf('setTimeout');
+  const setTimeoutEnd = fnBody.indexOf('}, 260)', setTimeoutIdx);
+  const setTimeoutBody = fnBody.substring(setTimeoutIdx, setTimeoutEnd);
+
+  assert.ok(!setTimeoutBody.includes('_openTerminal'),
+    '_openTerminal must NOT be inside the 260ms setTimeout — causes race condition with /connect POST');
+
+  // _openTerminal must appear AFTER the /connect POST
+  const connectIdx = fnBody.indexOf('/api/sessions/');
+  const openTermIdx = fnBody.indexOf('_openTerminal', connectIdx);
+  assert.ok(openTermIdx > connectIdx,
+    '_openTerminal must appear AFTER the /connect POST in the source');
+});
+