src/Plugin/CKEditor4To5Upgrade/Imce.php     |  62 +++++++++
 tests/src/Kernel/CKE4to5UpgradePathTest.php | 193 ++++++++++++++++++++++++++++
 2 files changed, 255 insertions(+)

diff --git a/src/Plugin/CKEditor4To5Upgrade/Imce.php b/src/Plugin/CKEditor4To5Upgrade/Imce.php
new file mode 100644
index 0000000..9373b41
--- /dev/null
+++ b/src/Plugin/CKEditor4To5Upgrade/Imce.php
@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\imce\Plugin\CKEditor4To5Upgrade;
+
+use Drupal\ckeditor5\HTMLRestrictions;
+use Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginInterface;
+use Drupal\Core\Plugin\PluginBase;
+use Drupal\filter\FilterFormatInterface;
+
+/**
+ * Provides the CKEditor 4 to 5 upgrade for IMCE module's CKEditor 4 plugins.
+ *
+ * @CKEditor4To5Upgrade(
+ *   id = "imce",
+ *   cke4_buttons = {
+ *     "ImceImage",
+ *     "ImceLink",
+ *   },
+ *   cke4_plugin_settings = {
+ *   },
+ *   cke5_plugin_elements_subset_configuration = {
+ *   }
+ * )
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+class Imce extends PluginBase implements CKEditor4To5UpgradePluginInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button, HTMLRestrictions $text_format_html_restrictions): ?array {
+    switch ($cke4_button) {
+      case 'ImceImage':
+        return ['imce_image'];
+
+      case 'ImceLink':
+        return ['imce_link'];
+
+      default:
+        throw new \OutOfBoundsException();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mapCKEditor4SettingsToCKEditor5Configuration(string $cke4_plugin_id, array $cke4_plugin_settings): ?array {
+    throw new \OutOfBoundsException();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function computeCKEditor5PluginSubsetConfiguration(string $cke5_plugin_id, FilterFormatInterface $text_format): ?array {
+    throw new \OutOfBoundsException();
+  }
+
+}
diff --git a/tests/src/Kernel/CKE4to5UpgradePathTest.php b/tests/src/Kernel/CKE4to5UpgradePathTest.php
new file mode 100644
index 0000000..4b91b77
--- /dev/null
+++ b/tests/src/Kernel/CKE4to5UpgradePathTest.php
@@ -0,0 +1,193 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\Tests\imce\Kernel;
+
+use Drupal\editor\Entity\Editor;
+use Drupal\filter\Entity\FilterFormat;
+use Drupal\Tests\ckeditor5\Kernel\SmartDefaultSettingsTest;
+
+/**
+ * @covers \Drupal\imce\Plugin\CKEditor4To5Upgrade\Imce
+ * @group imce
+ * @group ckeditor5
+ * @requires module ckeditor5
+ * @internal
+ */
+class CKE4to5UpgradePathTest extends SmartDefaultSettingsTest {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'imce',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    FilterFormat::create([
+      'format' => 'both',
+      'name' => 'Both IMCE buttons',
+      'filters' => [
+        'filter_html' => [
+          'status' => 1,
+          'settings' => [
+            'allowed_html' => '<p> <br> <strong> <h2> <h3> <img src alt height width data-entity-type data-entity-uuid> <a href>',
+          ],
+        ],
+      ],
+    ])->setSyncing(TRUE)->save();
+    FilterFormat::create([
+      'format' => 'image_only',
+      'name' => 'Only the IMCE image button',
+      'filters' => [
+        'filter_html' => [
+          'status' => 1,
+          'settings' => [
+            'allowed_html' => '<p> <br> <strong> <h2> <h3> <img src alt height width data-entity-type data-entity-uuid>',
+          ],
+        ],
+      ],
+    ])->setSyncing(TRUE)->save();
+    FilterFormat::create([
+      'format' => 'link_only',
+      'name' => 'Only the IMCE link button',
+      'filters' => [
+        'filter_html' => [
+          'status' => 1,
+          'settings' => [
+            'allowed_html' => '<p> <br> <strong> <h2> <h3> <a href>',
+          ],
+        ],
+      ],
+    ])->setSyncing(TRUE)->save();
+
+    $generate_editor_settings = function (array $imce_buttons) {
+      return [
+        'toolbar' => [
+          'rows' => [
+            0 => [
+              [
+                'name' => 'Basic Formatting',
+                'items' => [
+                  'Bold',
+                  'Format',
+                ],
+              ],
+              [
+                'name' => 'IMCE buttons',
+                'items' => $imce_buttons,
+              ],
+            ],
+          ],
+        ],
+        'plugins' => [
+          // The CKEditor 4 plugin functionality has no settings.
+        ],
+      ];
+    };
+
+    Editor::create([
+      'format' => 'both',
+      'editor' => 'ckeditor',
+      'settings' => $generate_editor_settings([
+        'ImceImage',
+        'ImceLink',
+      ]),
+    ])->setSyncing(TRUE)->save();
+    Editor::create([
+      'format' => 'image_only',
+      'editor' => 'ckeditor',
+      'settings' => $generate_editor_settings([
+        'ImceImage',
+      ]),
+    ])->setSyncing(TRUE)->save();
+    Editor::create([
+      'format' => 'link_only',
+      'editor' => 'ckeditor',
+      'settings' => $generate_editor_settings([
+        'ImceLink',
+      ]),
+    ])->setSyncing(TRUE)->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function provider() {
+    parent::provider();
+
+    $expected_ckeditor5_plugin_settings = [
+      'plugins' => [
+        'ckeditor5_heading' => [
+          'enabled_headings' => [
+            'heading2',
+            'heading3',
+          ],
+        ],
+      ],
+    ];
+
+    yield "both IMCE buttons" => [
+      'format_id' => 'both',
+      'filters_to_drop' => [],
+      'expected_ckeditor5_settings' => [
+        'toolbar' => [
+          'items' => [
+            'bold',
+            'heading',
+            '|',
+            'imce_image',
+            'imce_link',
+          ],
+        ],
+      ] + $expected_ckeditor5_plugin_settings,
+      'expected_superset' => '',
+      'expected_fundamental_compatibility_violations' => [],
+      'expected_db_logs' => [],
+      'expected_messages' => [],
+    ];
+    yield "IMCE image button only" => [
+      'format_id' => 'image_only',
+      'filters_to_drop' => [],
+      'expected_ckeditor5_settings' => [
+        'toolbar' => [
+          'items' => [
+            'bold',
+            'heading',
+            '|',
+            'imce_image',
+          ],
+        ],
+      ] + $expected_ckeditor5_plugin_settings,
+      'expected_superset' => '',
+      'expected_fundamental_compatibility_violations' => [],
+      'expected_db_logs' => [],
+      'expected_messages' => [],
+    ];
+    yield "IMCE link button only" => [
+      'format_id' => 'link_only',
+      'filters_to_drop' => [],
+      'expected_ckeditor5_settings' => [
+        'toolbar' => [
+          'items' => [
+            'bold',
+            'heading',
+            '|',
+            'imce_link',
+          ],
+        ],
+      ] + $expected_ckeditor5_plugin_settings,
+      'expected_superset' => '',
+      'expected_fundamental_compatibility_violations' => [],
+      'expected_db_logs' => [],
+      'expected_messages' => [],
+    ];
+  }
+
+}