Issue #2598488 by NickWilde: Views Page display menu expanded option is not included and gets destroyed by cache-rebuild

core/modules/views/config/schema/views.display.schema.yml

@@ -43,6 +43,9 @@ views.display.page:
         context:
           type: string
           label: 'Context'
+        expanded:
+          type: boolean
+          label: 'Expanded'
     tab_options:
       type: mapping
       label: 'Tab options'

core/modules/views/src/Plugin/Menu/ViewsMenuLink.php

@@ -127,6 +127,14 @@ class ViewsMenuLink extends MenuLinkBase implements ContainerFactoryPluginInterf
     return $this->loadView()->display_handler->getOption('menu')['description'];
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function isExpanded() {
+    return (bool) $this->loadView()->display_handler->getOption('menu')['expanded'];
+  }
+
+
   /**
    * {@inheritdoc}
    */

core/modules/views/src/Plugin/views/display/Page.php

@@ -132,6 +132,7 @@ class Page extends PathPluginBase {
         'menu_name' => array('default' => 'main'),
         'parent' => array('default' => ''),
         'context' => array('default' => ''),
+        'expanded' => array('default' => FALSE),
       ),
     );
     $options['tab_options'] = array(
@@ -239,7 +240,7 @@ class Page extends PathPluginBase {
         );
         $menu = $this->getOption('menu');
         if (empty($menu)) {
-          $menu = array('type' => 'none', 'title' => '', 'weight' => 0);
+          $menu = array('type' => 'none', 'title' => '', 'weight' => 0, 'expanded' => FALSE);
         }
         $form['menu']['type'] = array(
           '#prefix' => '<div class="views-left-30">',
@@ -293,6 +294,12 @@ class Page extends PathPluginBase {
             ),
           ),
         );
+        $form['menu']['expanded'] = [
+          '#title' => $this->t('Show as expanded'),
+          '#type' => 'checkbox',
+          '#default_value' => !empty($menu['expanded']),
+          '#description' => $this->t('If selected and this menu link has children, the menu will always appear expanded. '),
+        ];
 
         // Only display the parent selector if Menu UI module is enabled.
         $menu_parent = $menu['menu_name'] . ':' . $menu['parent'];

core/modules/views/src/Tests/Plugin/DisplayPageTest.php

@@ -136,6 +136,8 @@ class DisplayPageTest extends ViewKernelTestBase {
     $this->assertTrue(isset($tree['system.admin']->subtree['views_view:views.test_page_display_menu.page_4']));
     $menu_link = $tree['system.admin']->subtree['views_view:views.test_page_display_menu.page_4']->link;
     $this->assertEqual($menu_link->getTitle(), 'Test child (with parent)');
+    $this->assertEqual($menu_link->isExpanded(), TRUE);
+    $this->assertEqual($menu_link->getDescription(), 'Sample description.');
   }
 
   /**

core/modules/views/tests/modules/views_test_config/test_views/views.view.test_page_display_menu.yml

@@ -91,10 +91,11 @@ display:
         type: normal
         title: 'Test child (with parent)'
         parent: system.admin
-        description: ''
+        description: 'Sample description.'
         menu_name: admin
         weight: 0
         context: '0'
+        expanded: true
       defaults:
         title: false
     display_plugin: page