.../Drupal/system/Controller/SystemController.php  |   10 +-
 .../Tests/Controller/SystemControllerTest.php      |  347 ++++++++++++++++++++
 2 files changed, 351 insertions(+), 6 deletions(-)

diff --git a/core/modules/system/lib/Drupal/system/Controller/SystemController.php b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
index e16c838..d07752c 100644
--- a/core/modules/system/lib/Drupal/system/Controller/SystemController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
@@ -434,13 +434,11 @@ public static function setLinkActiveClass(array $element, array $context) {
         $class .= 'active';
         $node->setAttribute('class', $class);
 
-        $body_dom_node = $dom->getElementsByTagName('body')->item(0);
-        $updated_tag = substr($dom->saveHTML($body_dom_node), 6, -7);
+        // Get the updated tag.
+        $updated_tag = $dom->saveXML($node, LIBXML_NOEMPTYTAG);
+        // saveXML() added a closing tag, remove it.
+        $updated_tag = substr($updated_tag, 0, strrpos($updated_tag, '<'));
 
-        // If saveHTML() added a closing tag (it does so for <a>), remove it.
-        if (substr_count($updated_tag, '<') > 1) {
-          $updated_tag = substr($updated_tag, 0, strrpos($updated_tag, '<'));
-        }
         $element['#markup'] = str_replace($tag, $updated_tag, $element['#markup']);
 
         // Ensure we only search the remaining HTML.
diff --git a/core/modules/system/tests/Drupal/system/Tests/Controller/SystemControllerTest.php b/core/modules/system/tests/Drupal/system/Tests/Controller/SystemControllerTest.php
new file mode 100644
index 0000000..82c694e
--- /dev/null
+++ b/core/modules/system/tests/Drupal/system/Tests/Controller/SystemControllerTest.php
@@ -0,0 +1,347 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\edit\Tests\Access\EditEntityAccessCheckTest.
+ */
+
+namespace Drupal\edit\Tests\Access;
+
+use Symfony\Component\HttpFoundation\Request;
+use Drupal\Component\Utility\Json;
+use Drupal\Core\Template\Attribute;
+use Drupal\system\Controller\SystemController;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the System controller's #post_render_cache callback for active links.
+ *
+ * @group Drupal
+ * @group System
+ *
+ * @see \Drupal\system\Controller\SystemController::setLinkActiveClass()
+ */
+class SystemControllerTest extends UnitTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'System controller set active link class test',
+      'description' => 'Unit test of system controller #post_render_cache callback for marking active links.',
+      'group' => 'System'
+    );
+  }
+
+  /**
+   * Provides test data for testSetLinkActiveClass().
+   *
+   * @see \Drupal\system\Controller\SystemController::setLinkActiveClass()
+   */
+  public function providerTestSetLinkActiveClass() {
+    // Define all the variations that *don't* affect whether or not an "active"
+    // class is set, but that should remain unchanged:
+    // - surrounding HTML
+    // - tags for which to test the setting of the "active" class
+    // - content of said tags
+    $edge_case_html5 = '<audio src="foo.ogg">
+  <track kind="captions" src="foo.en.vtt" srclang="en" label="English">
+  <track kind="captions" src="foo.sv.vtt" srclang="sv" label="Svenska">
+</audio>';
+    $html = array(
+      // Simple HTML.
+      0 => array('prefix' => '<div><p>', 'suffix' => '</p></div>'),
+      // Tricky HTML5 example that's unsupported by PHP <=5.4's DOMDocument:
+      // https://drupal.org/comment/7938201#comment-7938201.
+      1 => array('prefix' => '<div><p>', 'suffix' => '</p>' . $edge_case_html5 . '</div>'),
+      // Multi-byte content *before* the HTML that needs the "active" class.
+      2 => array('prefix' => '<div><p>αβγδεζηθικλμνξοσὠ</p><p>', 'suffix' => '</p></div>'),
+    );
+    $tags = array(
+      // Of course, it must work on anchors.
+      'a',
+      // Unfortunately, it must also work on list items.
+      'li',
+      // … and therefor, on *any* tag, really.
+      'foo',
+    );
+    $contents = array(
+      // Regular content.
+      'test',
+      // Mix of UTF-8 and HTML entities, both must be retained.
+      '☆ 3 × 4 = €12 and 4 &times; 3 = &euro;12 &#9734',
+      // Multi-byte content.
+      'ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ',
+      // Text that closely approximates an important attribute, but should be
+      // ignored.
+      'data-drupal-link-system-path=&quot;&lt;front&gt;&quot;',
+    );
+
+    // Define all variations that *do* affect whether or not an "active" class
+    // is set: all possible situations that can be encountered.
+    $situations = array();
+
+    // Situations with context: front page, English, no query.
+    $context = array(
+      'path' => 'myfrontpage',
+      'front' => TRUE,
+      'language' => 'en',
+      'query' => array(),
+    );
+    // Nothing to do.
+    $markup = '<foo>bar</foo>';
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => array());
+    // Matching path, plus all matching variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => 'myfrontpage',
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    // Matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => TRUE));
+    // Special matching path, plus all variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => '<front>',
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    // Special matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => TRUE));
+
+    // Situations with context: non-front page, Dutch, no query.
+    $context = array(
+      'path' => 'llama',
+      'front' => FALSE,
+      'language' => 'nl',
+      'query' => array(),
+    );
+    // Nothing to do.
+    $markup = '<foo>bar</foo>';
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => array());
+    // Matching path, plus all matching variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => 'llama',
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    // Matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => TRUE));
+    // Special non-matching path, plus all variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => '<front>',
+    );
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => '{"foo":"bar"}'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => TRUE));
+
+    // Situations with context: non-front page, Dutch, with query.
+    $context = array(
+      'path' => 'llama',
+      'front' => FALSE,
+      'language' => 'nl',
+      'query' => array('foo' => 'bar'),
+    );
+    // Nothing to do.
+    $markup = '<foo>bar</foo>';
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => array());
+    // Matching path, plus all matching variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => 'llama',
+      'data-drupal-link-query' => Json::encode(array('foo' => 'bar')),
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    // Matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    unset($attributes['data-drupal-link-query']);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => TRUE));
+    // Special non-matching path, plus all variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => '<front>',
+    );
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    unset($attributes['data-drupal-link-query']);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => TRUE));
+
+    // Situations with context: non-front page, Dutch, with query.
+    $context = array(
+      'path' => 'llama',
+      'front' => FALSE,
+      'language' => 'nl',
+      'query' => array('foo' => 'bar'),
+    );
+    // Nothing to do.
+    $markup = '<foo>bar</foo>';
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => array());
+    // Matching path, plus all matching variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => 'llama',
+      'data-drupal-link-query' => Json::encode(array('foo' => 'bar')),
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    // Matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    unset($attributes['data-drupal-link-query']);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => TRUE));
+    // Special non-matching path, plus all variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => '<front>',
+    );
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    unset($attributes['data-drupal-link-query']);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl', 'data-drupal-link-query' => TRUE));
+
+    // Situations with context: front page, English, query.
+    $context = array(
+      'path' => 'myfrontpage',
+      'front' => TRUE,
+      'language' => 'en',
+      'query' => array('foo' => 'bar'),
+    );
+    // Nothing to do.
+    $markup = '<foo>bar</foo>';
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => array());
+    // Matching path, plus all matching variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => 'myfrontpage',
+      'data-drupal-link-query' => Json::encode(array('foo' => 'bar')),
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    // Matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    unset($attributes['data-drupal-link-query']);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => TRUE));
+    // Special matching path, plus all variations.
+    $attributes = array(
+      'data-drupal-link-system-path' => '<front>',
+      'data-drupal-link-query' => Json::encode(array('foo' => 'bar')),
+    );
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes);
+    $situations[] = array('context' => $context, 'is active' => TRUE, 'attributes' => $attributes + array('hreflang' => 'en'));
+    // Special matching path, plus all non-matching variations.
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'nl'));
+    unset($attributes['data-drupal-link-query']);
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('data-drupal-link-query' => TRUE));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => ""));
+    $situations[] = array('context' => $context, 'is active' => FALSE, 'attributes' => $attributes + array('hreflang' => 'en', 'data-drupal-link-query' => TRUE));
+
+    // Helper function to generate a stubbed renderable array.
+    $create_element = function ($markup) {
+      return array(
+        '#markup' => $markup,
+        '#attached' => array(),
+      );
+    };
+
+    // Loop over the surrounding HTML variations.
+    $data = array();
+    for ($h = 0; $h < count($html); $h++) {
+      $html_prefix = $html[$h]['prefix'];
+      $html_suffix = $html[$h]['suffix'];
+      // Loop over the tag variations.
+      for ($t = 0; $t < count($tags); $t++) {
+        $tag = $tags[$t];
+        // Loop over the tag contents variations.
+        for ($c = 0; $c < count($contents); $c++) {
+          $tag_content = $contents[$c];
+
+          $create_markup = function (Attribute $attributes) use ($html_prefix, $html_suffix, $tag, $tag_content) {
+            return $html_prefix . '<' . $tag . $attributes . '>' . $tag_content . '</' . $tag . '>' . $html_suffix;
+          };
+
+          // Loop over the situations.
+          for ($s = 0; $s < count($situations); $s++) {
+            $situation = $situations[$s];
+
+            // Build the source markup.
+            $source_markup = $create_markup(new Attribute($situation['attributes']));
+
+            // Build the target markup. If no "active" class should be set, the
+            // resulting HTML should be identical. Otherwise, it should get an
+            // "active" class, either by extending an existing "class" attribute
+            // or by adding a "class" attribute.
+            $target_markup = NULL;
+            if (!$situation['is active']) {
+              $target_markup = $source_markup;
+            }
+            else {
+              $active_attributes = $situation['attributes'];
+              if (!isset($active_attributes['class'])) {
+                $active_attributes['class'] = array();
+              }
+              $active_attributes['class'][] = 'active';
+              $target_markup = $create_markup(new Attribute($active_attributes));
+            }
+
+            $data[] = array($create_element($source_markup), $situation['context'], $create_element($target_markup));
+          }
+        }
+      }
+    }
+
+    return $data;
+  }
+
+  /**
+   * Tests setLinkActiveClass().
+   *
+   * @param array $element
+   *  A renderable array with the following keys:
+   *    - #markup
+   *    - #attached
+   * @param array $context
+   *   The page context to simulate. An array with the following keys:
+   *   - path: the system path of the currently active page
+   *   - front: whether the current page is the front page (which implies the
+   *     current path might also be <front>)
+   *   - language: the language code of the currently active page
+   *   - query: the query string for the currently active page
+   * @param array $expected_element
+   *   The returned renderable array.
+   *
+   * @dataProvider providerTestSetLinkActiveClass
+   */
+  public function testSetLinkActiveClass(array $element, array $context, $expected_element) {
+    $this->assertSame($expected_element, SystemController::setLinkActiveClass($element, $context));
+  }
+
+}