Issue #2990517 by tim.plunkett, alphex, xjm, phenaproxima, neclimdul: Adding a display mode to a content type using layout, and disabling layout on that new display mode removes the layout_builder__layout field and breaks layout in already configured display modes

core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php

@@ -112,8 +112,8 @@ class LayoutBuilderEntityViewDisplay extends BaseEntityViewDisplay implements La
       if ($new_value) {
         $this->addSectionField($entity_type_id, $bundle, 'layout_builder__layout');
       }
-      elseif ($field = FieldConfig::loadByName($entity_type_id, $bundle, 'layout_builder__layout')) {
-        $field->delete();
+      else {
+        $this->removeSectionField($entity_type_id, $bundle, 'layout_builder__layout');
       }
     }
 
@@ -139,6 +139,31 @@ class LayoutBuilderEntityViewDisplay extends BaseEntityViewDisplay implements La
     }
   }
 
+  /**
+   * Removes a layout section field if it is no longer needed.
+   *
+   * Because the field is shared across all view modes, the field will only be
+   * removed if no other view modes are using it.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle
+   *   The bundle.
+   * @param string $field_name
+   *   The name for the layout section field.
+   */
+  protected function removeSectionField($entity_type_id, $bundle, $field_name) {
+    $query = $this->entityTypeManager()->getStorage($this->getEntityTypeId())->getQuery()
+      ->condition('targetEntityType', $this->getTargetEntityTypeId())
+      ->condition('bundle', $this->getTargetBundle())
+      ->condition('mode', $this->getMode(), '<>')
+      ->condition('third_party_settings.layout_builder.allow_custom', TRUE);
+    $enabled = (bool) $query->count()->execute();
+    if (!$enabled && $field = FieldConfig::loadByName($entity_type_id, $bundle, $field_name)) {
+      $field->delete();
+    }
+  }
+
   /**
    * Adds a layout section field to a given bundle.
    *

core/modules/layout_builder/tests/src/Functional/LayoutDisplayTest.php

@@ -0,0 +1,94 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests functionality of the entity view display with regard to Layout Builder.
+ *
+ * @group layout_builder
+ */
+class LayoutDisplayTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['field_ui', 'layout_builder', 'block', 'node'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    // @todo The Layout Builder UI relies on local tasks; fix in
+    //   https://www.drupal.org/project/drupal/issues/2917777.
+    $this->drupalPlaceBlock('local_tasks_block');
+
+    $this->createContentType([
+      'type' => 'bundle_with_section_field',
+    ]);
+    $this->createNode(['type' => 'bundle_with_section_field']);
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'configure any layout',
+      'administer node display',
+      'administer display modes',
+    ], 'foobar'));
+  }
+
+  /**
+   * Tests the interaction between multiple view modes.
+   */
+  public function testMultipleViewModes() {
+    $assert_session = $this->assertSession();
+    $page = $this->getSession()->getPage();
+    $field_ui_prefix = 'admin/structure/types/manage/bundle_with_section_field/display';
+
+    // Enable Layout Builder for the default view modes, and overrides.
+    $this->drupalGet("$field_ui_prefix/default");
+    $page->checkField('layout[enabled]');
+    $page->pressButton('Save');
+    $page->checkField('layout[allow_custom]');
+    $page->pressButton('Save');
+
+    $this->drupalGet('node/1');
+    $assert_session->pageTextNotContains('Powered by Drupal');
+
+    $assert_session->linkExists('Layout');
+    $this->clickLink('Layout');
+    $assert_session->linkExists('Add Block');
+    $this->clickLink('Add Block');
+    $assert_session->linkExists('Powered by Drupal');
+    $this->clickLink('Powered by Drupal');
+    $page->pressButton('Add Block');
+    $assert_session->linkExists('Save Layout');
+    $this->clickLink('Save Layout');
+    $assert_session->pageTextContains('Powered by Drupal');
+
+    // Add a new view mode.
+    $this->drupalGet('admin/structure/display-modes/view/add/node');
+    $page->fillField('label', 'New');
+    $page->fillField('id', 'new');
+    $page->pressButton('Save');
+
+    // Enable the new view mode.
+    $this->drupalGet("$field_ui_prefix/default");
+    $page->checkField('display_modes_custom[new]');
+    $page->pressButton('Save');
+
+    // Enable and disable Layout Builder for the new view mode.
+    $this->drupalGet("$field_ui_prefix/new");
+    $page->checkField('layout[enabled]');
+    $page->pressButton('Save');
+    $page->uncheckField('layout[enabled]');
+    $page->pressButton('Save');
+    $page->pressButton('Confirm');
+
+    // The node using the default view mode still contains its overrides.
+    $this->drupalGet('node/1');
+    $assert_session->pageTextContains('Powered by Drupal');
+  }
+
+}