fix(web): prevent browser context menu over task menus (#1627)

ui/flutter/test/support/context_menu_event.dart

@@ -0,0 +1 @@
+export 'context_menu_event_stub.dart' if (dart.library.js_interop) 'context_menu_event_web.dart';

ui/flutter/test/support/context_menu_event_stub.dart

@@ -0,0 +1,5 @@
+import 'dart:ui';
+
+bool dispatchContextMenu(Offset position, int viewId) {
+  throw UnsupportedError('DOM contextmenu events require a browser');
+}

ui/flutter/test/support/context_menu_event_web.dart

@@ -0,0 +1,25 @@
+import 'dart:ui';
+import 'dart:ui_web' as ui_web;
+
+import 'package:web/web.dart' as web;
+
+/// Exercises DOM cancellation, independently of Flutter's global menu flag.
+/// Synthetic events cannot prove whether a browser's native menu is visible.
+bool dispatchContextMenu(Offset position, int viewId) {
+  final view = PlatformDispatcher.instance.views.singleWhere((view) => view.viewId == viewId);
+  final host = ui_web.views.getHostElement(viewId) as web.HTMLElement;
+  final bounds = host.getBoundingClientRect();
+  final logicalSize = view.physicalSize / view.devicePixelRatio;
+  final event = web.MouseEvent(
+    'contextmenu',
+    web.MouseEventInit(
+      bubbles: true,
+      cancelable: true,
+      button: 2,
+      clientX: (bounds.left + position.dx * bounds.width / logicalSize.width).round(),
+      clientY: (bounds.top + position.dy * bounds.height / logicalSize.height).round(),
+    ),
+  );
+  host.dispatchEvent(event);
+  return event.defaultPrevented;
+}