Issue #2631468 by snufkin, johnmcc, DamienMcKenna, aaronbauman, kiwad,...

Issue #2631468 by snufkin, johnmcc, DamienMcKenna, aaronbauman, kiwad, acrosman, esolitos, Tom Robert, Yogesh Pawar, acke: Menu subtrees in menu blocks show all subitems regardless of the active menu item

core/modules/system/src/Plugin/Block/SystemMenuBlock.php

@@ -135,6 +135,25 @@ public function build() {
       $parameters->setMaxDepth(min($level + $depth - 1, $this->menuTree->maxDepth()));
     }
 
+    // For menu blocks with start level greater than 1, only show menu items
+    // from the current active trail. Adjust the root according to the current
+    // position in the menu in order to determine if we can show the subtree.
+    if ($level > 1) {
+      if (count($parameters->activeTrail) >= $level) {
+        // Active trail array is child-first. Reverse it, and pull the new menu
+        // root based on the parent of the configured start level.
+        $menu_trail_ids = array_reverse(array_values($parameters->activeTrail));
+        $menu_root = $menu_trail_ids[$level - 1];
+        $parameters->setRoot($menu_root)->setMinDepth(1);
+        if ($depth > 0) {
+          $parameters->setMaxDepth(min($level - 1 + $depth - 1, $this->menuTree->maxDepth()));
+        }
+      }
+      else {
+        return [];
+      }
+    }
+
     $tree = $this->menuTree->load($menu_name, $parameters);
     $manipulators = [
       ['callable' => 'menu.default_tree_manipulators:checkAccess'],

core/modules/system/tests/src/Kernel/Block/SystemMenuBlockTest.php

@@ -218,9 +218,7 @@ public function testConfigLevelDepth() {
       'test.example6' => [],
       'test.example8' => [],
     ];
-    $no_active_trail_expectations['level_2_only'] = [
-      'test.example7' => [],
-    ];
+    $no_active_trail_expectations['level_2_only'] = [];
     $no_active_trail_expectations['level_3_only'] = [];
     $no_active_trail_expectations['level_1_and_beyond'] = $no_active_trail_expectations['all'];
     $no_active_trail_expectations['level_2_and_beyond'] = $no_active_trail_expectations['level_2_only'];
@@ -266,7 +264,6 @@ public function testConfigLevelDepth() {
     ];
     $active_trail_expectations['level_2_only'] = [
       'test.example3' => [],
-      'test.example7' => [],
     ];
     $active_trail_expectations['level_3_only'] = [
       'test.example4' => [],
@@ -276,7 +273,6 @@ public function testConfigLevelDepth() {
       'test.example3' => [
         'test.example4' => [],
       ],
-      'test.example7' => [],
     ];
     $active_trail_expectations['level_3_and_beyond'] = $active_trail_expectations['level_3_only'];
     foreach ($blocks as $id => $block) {