Use switch widget instead of dismiss

app/lib/frontend/templates/views/shared/layout.dart

@@ -201,9 +201,11 @@ d.Node pageLayoutNode({
                       d.div(
                         classes: ['dismisser'],
                         attributes: {
-                          'data-widget': 'dismiss',
-                          'data-dismiss-target': '.announcement-banner',
-                          'data-dismiss-message-id': announcementBannerHash,
+                          'data-widget': 'switch',
+                          'data-switch-target': '.announcement-banner',
+                          'data-switch-enabled': 'dismissed',
+                          'data-switch-initial-state': 'false',
+                          'data-switch-state-id': announcementBannerHash,
                         },
                         text: 'x',
                       ),

pkg/web_app/lib/src/widget/switch/widget.dart

@@ -0,0 +1,127 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:js_interop';
+
+import 'package:collection/collection.dart';
+import 'package:web/web.dart';
+
+import '../../web_util.dart';
+
+/// Create a switch widget on [element].
+///
+/// A `data-switch-target` is required, this must be a CSS selector for the
+/// element(s) that are affected when this widget is clicked.
+///
+/// The optional `data-switch-initial-state` property may be used to provide an
+/// initial state. The initial state is used if state can be loaded from
+/// `localStorage` (because there is none). If not provided, initial state is
+/// derived from document state.
+///
+/// The optional `data-switch-state-id` property may be used to provide an
+/// identifier for the sttae of this switch in `localStorage`. If supplied state
+/// will be sync'ed across windows.
+///
+/// The optional `data-switch-enabled` property may be used to specify a space
+/// separated list of classes to be applied to `data-switch-target` when the
+/// switch is enabled.
+///
+/// The optional `data-switch-disabled` property may be used to specify a space
+/// separated list of classes to be applied to `data-switch-target` when the
+/// switch is disabled.
+void create(HTMLElement element, Map<String, String> options) {
+  final target = options['target'];
+  if (target == null) {
+    throw UnsupportedError('data-switch-target required');
+  }
+  final initialState_ = options['initial-state'];
+  final stateId = options['state-id'];
+  final enabledClassList = (options['enabled'] ?? '')
+      .split(' ')
+      .where((s) => s.isNotEmpty)
+      .toSet()
+      .toList();
+  final disabledClassList = (options['disabled'] ?? '')
+      .split(' ')
+      .where((s) => s.isNotEmpty)
+      .toSet()
+      .toList();
+
+  void applyState(bool enabled) {
+    for (final e in document.querySelectorAll(target).toList()) {
+      if (e.isA<HTMLElement>()) {
+        final element = e as HTMLHtmlElement;
+        if (enabled) {
+          for (final c in enabledClassList) {
+            element.classList.add(c);
+          }
+          for (final c in disabledClassList) {
+            element.classList.remove(c);
+          }
+        } else {
+          for (final c in enabledClassList) {
+            element.classList.remove(c);
+          }
+          for (final c in disabledClassList) {
+            element.classList.add(c);
+          }
+        }
+      }
+    }
+  }
+
+  bool? initialState;
+  if (stateId != null) {
+    void onStorage(StorageEvent e) {
+      if (e.key == 'switch-$stateId' && e.storageArea == window.localStorage) {
+        applyState(e.newValue == 'true');
+      }
+    }
+
+    window.addEventListener('storage', onStorage.toJS);
+    final state = window.localStorage.getItem('switch-$stateId');
+    if (state != null) {
+      initialState = state == 'true';
+    }
+  }
+
+  // Set initialState, if present
+  if (initialState_ != null) {
+    initialState ??= initialState_ == 'true';
+  }
+  // If there are no classes to be applied, this is weird, but then we can't
+  // infer an initial state.
+  if (enabledClassList.isEmpty && disabledClassList.isEmpty) {
+    initialState ??= false;
+  }
+  // Infer initial state from document state, unless loaded from localStorage
+  initialState ??= document
+      .querySelectorAll(target)
+      .toList()
+      .where((e) => e.isA<HTMLElement>())
+      .map((e) => e as HTMLElement)
+      .every(
+        (e) =>
+            enabledClassList.every((c) => e.classList.contains(c)) &&
+            disabledClassList.none((c) => e.classList.contains(c)),
+      );
+
+  applyState(initialState);
+  var state = initialState;
+
+  void onClick(MouseEvent e) {
+    if (e.defaultPrevented) {
+      return;
+    }
+    e.preventDefault();
+
+    state = !state;
+    applyState(state);
+    if (stateId != null) {
+      window.localStorage.setItem('switch-$stateId', state.toString());
+    }
+  }
+
+  element.addEventListener('click', onClick.toJS);
+}

pkg/web_app/lib/src/widget/widget.dart

@@ -10,7 +10,7 @@ import 'package:web/web.dart';
 
 import '../web_util.dart';
 import 'completion/widget.dart' deferred as completion;
-import 'dismiss/widget.dart' deferred as dismiss;
+import 'switch/widget.dart' deferred as switch_;
 
 /// Function to create an instance of the widget given an element and options.
 ///
@@ -32,7 +32,7 @@ typedef _WidgetLoaderFn = FutureOr<_WidgetFn> Function();
 /// Map from widget name to widget loader
 final _widgets = <String, _WidgetLoaderFn>{
   'completion': () => completion.loadLibrary().then((_) => completion.create),
-  'dismiss': () => dismiss.loadLibrary().then((_) => dismiss.create),
+  'switch': () => switch_.loadLibrary().then((_) => switch_.create),
 };
 
 Future<_WidgetFn> _noSuchWidget() async =>

pkg/web_app/test/deferred_import_test.dart

@@ -37,6 +37,7 @@ void main() {
       ],
       'completion/': [],
       'dismiss/': [],
+      'switch/': [],
     };
 
     for (final file in files) {

pkg/web_css/lib/src/_base.scss

@@ -400,7 +400,7 @@ pre {
   }
 
   &.dismissed {
-    display: none;
+    visibility: hidden;
   }
 
   z-index: 0;