Fix prevent session loss when library is closed after main window (#11073)

src/browser/components/sessionstore/fix-library-session.patch

@@ -0,0 +1,16 @@
+diff --git a/browser/components/sessionstore/SessionStore.sys.mjs b/browser/components/sessionstore/SessionStore.sys.mjs
+--- a/browser/components/sessionstore/SessionStore.sys.mjs
++++ b/browser/components/sessionstore/SessionStore.sys.mjs
+@@ -2139,6 +2139,12 @@ var SessionStoreInternal = {
+     let closedWindowState = this._collectWindowData(aWindow);
+
+     if (closedWindowState) {
++      // Only save session state for actual browser windows (windowtype="navigator:browser")
++      // This prevents non-browser windows like Library (Places:Organizer) from affecting the session
++      if (aWindow.document.documentElement.getAttribute("windowtype") !== "navigator:browser") {
++        return;
++      }
++
+       let newWindowState;
+       if (
+         !lazy.SessionStartup.willRestore()

src/zen/tests/sessionstore/browser_library_window_session.js

@@ -0,0 +1,30 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+"use strict";
+
+add_task(async function test_library_window_session() {
+  // Open a new window with some tabs
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  let tab1 = await BrowserTestUtils.addTab(win.gBrowser, "about:mozilla");
+  let tab2 = await BrowserTestUtils.addTab(win.gBrowser, "about:config");
+
+  // Open the library window
+  let libraryWin = await BrowserTestUtils.promiseLibraryWindow();
+
+  // Close the main browser window
+  await BrowserTestUtils.closeWindow(win);
+
+  // Wait a bit to ensure session is saved
+  await TestUtils.waitForTick();
+
+  // Close library window
+  await BrowserTestUtils.closeWindow(libraryWin);
+
+  // Get the state file directly to verify it has content
+  let state = SessionFile.read();
+  ok(state, "Session file should exist");
+  ok(state.windows?.length > 0, "Session should have windows");
+  ok(state.windows[0].tabs?.length >= 2, "Session should have our test tabs");
+});