diff --git a/core/modules/outside_in/outside_in.libraries.yml b/core/modules/outside_in/outside_in.libraries.yml
index 8bdeb62..e99eef8 100644
--- a/core/modules/outside_in/outside_in.libraries.yml
+++ b/core/modules/outside_in/outside_in.libraries.yml
@@ -7,7 +7,9 @@ drupal.outside_in:
       css/outside_in.module.css: {}
       css/outside_in.motion.css: {}
     theme:
-      # Set the group higher than theme CSS so that it overrides jQuery UI.
+      # @todo Set the group higher than CSS_AGGREGATE_THEME so that it overrides
+      #   both jQuery UI and Classy's dialog.css, remove in
+      #   https://www.drupal.org/node/1945262.
       css/outside_in.theme.css: { group: 200 }
   dependencies:
     - core/jquery
diff --git a/core/modules/outside_in/outside_in.module b/core/modules/outside_in/outside_in.module
index 815f3ad..df8b84a 100644
--- a/core/modules/outside_in/outside_in.module
+++ b/core/modules/outside_in/outside_in.module
@@ -140,9 +140,11 @@ function outside_in_block_alter(&$definitions) {
  * Implements hook_css_alter().
  */
 function outside_in_css_alter(&$css, AttachedAssetsInterface $assets) {
-  // @todo Remove once custom groups in *.libraries.yml are respected.
+  // @todo Remove once conditional ordering is introduced in
+  //   https://www.drupal.org/node/1945262.
   $path = drupal_get_path('module', 'outside_in') . '/css/outside_in.theme.css';
   if (isset($css[$path])) {
+    // Use 200 to come after CSS_AGGREGATE_THEME.
     $css[$path]['group'] = 200;
   }
 }
diff --git a/core/modules/outside_in/src/Tests/Ajax/OffCanvasDialogTest.php b/core/modules/outside_in/src/Tests/Ajax/OffCanvasDialogTest.php
index 42c0ecf..3cc3acd 100644
--- a/core/modules/outside_in/src/Tests/Ajax/OffCanvasDialogTest.php
+++ b/core/modules/outside_in/src/Tests/Ajax/OffCanvasDialogTest.php
@@ -45,6 +45,7 @@ public function testDialog() {
           'resizable' => 'w',
           'draggable' => FALSE,
           'drupalAutoButtons' => FALSE,
+          'buttons' => [],
         ],
     ];
 
diff --git a/core/modules/outside_in/tests/src/Unit/Ajax/OpenOffCanvasDialogCommandTest.php b/core/modules/outside_in/tests/src/Unit/Ajax/OpenOffCanvasDialogCommandTest.php
index eac1b0f..76d570a 100644
--- a/core/modules/outside_in/tests/src/Unit/Ajax/OpenOffCanvasDialogCommandTest.php
+++ b/core/modules/outside_in/tests/src/Unit/Ajax/OpenOffCanvasDialogCommandTest.php
@@ -30,6 +30,7 @@ public function testRender() {
         'resizable' => 'w',
         'draggable' => FALSE,
         'drupalAutoButtons' => FALSE,
+        'buttons' => [],
       ],
     ];
     $this->assertEquals($expected, $command->render());