fix(widgets): never quit the app from a caption given its window

lib/src/widgets/window_caption.dart

@@ -104,7 +104,12 @@ class _WindowCaptionState extends State<WindowCaption> {
               () => nativeapi.Window.isCloseSupported() && window.close(),
             ) ??
             false);
-    if (!closed) {
+    // Window.close() returns true once the request is made; a veto arrives
+    // asynchronously, so false means the request could not be made. Only a
+    // caption that follows the current window may then quit the app: a
+    // caption given its window must not end the whole application, which may
+    // have other windows.
+    if (!closed && widget.window == null) {
       tryNative(() => nativeapi.Application.instance.quit(0));
     }
   }

test/window_caption_close_test.dart

@@ -0,0 +1,45 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:window_manager/window_manager.dart';
+
+// A window whose close request cannot be made, as when the platform refuses
+// it. No native window is created.
+class _RefusingWindow extends Window {
+  _RefusingWindow() : super.borrowed(0);
+
+  int closes = 0;
+
+  @override
+  bool get isMaximized => false;
+
+  @override
+  bool close() {
+    closes++;
+    return false;
+  }
+}
+
+void main() {
+  testWidgets('close button of an explicit window only addresses that window', (
+    tester,
+  ) async {
+    final window = _RefusingWindow();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: SizedBox(
+          height: kWindowCaptionHeight,
+          child: WindowCaption(window: window),
+        ),
+      ),
+    );
+
+    await tester.tap(find.byType(WindowCaptionButton).last);
+    await tester.pump();
+
+    // The request went to the given window and nothing else was attempted:
+    // with an explicit window the caption no longer falls back to quitting
+    // the application when the window does not close.
+    expect(window.closes, Window.isCloseSupported() ? 1 : 0);
+    expect(tester.takeException(), isNull);
+  });
+}