Issue #2809305 by Upchuk, Pavan B S, Jo Fitzgerald, tim.plunkett, Berdir: Block Context assignment form element shows even if no options are available

(cherry picked from commit 55f13aa)

Author: alexpott

lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php

@@ -48,7 +48,10 @@ protected function addContextAssignmentElement(ContextAwarePluginInterface $plug
         ];
       }
 
-      if (count($options) > 1 || !$definition->isRequired()) {
+      // Show the context selector only if there is more than 1 option to choose
+      // from. Also, show if there is a single option but the plugin does not
+      // require a context.
+      if (count($options) > 1 || (count($options) == 1 && !$definition->isRequired())) {
         $assignments = $plugin->getContextMapping();
         $element[$context_slot] = [
           '#title' => $definition->getLabel() ?: $this->t('Select a @context value:', ['@context' => $context_slot]),

modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareNoValidContextOptionsBlock.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Drupal\block_test\Plugin\Block;
+
+use Drupal\Core\Block\BlockBase;
+
+/**
+ * Provides a context-aware block that uses a not-passed, non-required context.
+ *
+ * @Block(
+ *   id = "test_context_aware_no_valid_context_options",
+ *   admin_label = @Translation("Test context-aware block - no valid context options"),
+ *   context_definitions = {
+ *     "email" = @ContextDefinition("email", required = FALSE)
+ *   }
+ * )
+ */
+class TestContextAwareNoValidContextOptionsBlock extends BlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    return [
+      '#markup' => 'Rendered block with no valid context options',
+    ];
+  }
+
+}

modules/block/tests/src/Functional/BlockUiTest.php

@@ -239,6 +239,13 @@ public function testContextAwareBlocks() {
     $this->assertText('User context found.');
     $this->assertRaw($expected_text);
 
+    // Test context mapping form element is not visible if there are no valid
+    // context options for the block (the test_context_aware_no_valid_context_options
+    // block has one context defined which is not available for it on the
+    // Block Layout interface).
+    $this->drupalGet('admin/structure/block/add/test_context_aware_no_valid_context_options/classy');
+    $this->assertSession()->fieldNotExists('edit-settings-context-mapping-email');
+
     // Test context mapping allows empty selection for optional contexts.
     $this->drupalGet('admin/structure/block/manage/testcontextawareblock');
     $edit = [