diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index c7848e2..89bbcf9 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -78,7 +78,7 @@ class ExtensionDiscovery {
   protected $profileDirectories;
 
   /**
-   * The app root.
+   * The app root for the current operation.
    *
    * @var string
    */
@@ -208,12 +208,12 @@ public function scan($type, $include_tests = NULL) {
     $files = array();
     foreach ($searchdirs as $dir) {
       // Discover all extensions in the directory, unless we did already.
-      if (!isset(static::$files[$dir][$include_tests])) {
-        static::$files[$dir][$include_tests] = $this->scanDirectory($dir, $include_tests);
+      if (!isset(static::$files[$this->root][$dir][$include_tests])) {
+        static::$files[$this->root][$dir][$include_tests] = $this->scanDirectory($dir, $include_tests);
       }
       // Only return extensions of the requested type.
-      if (isset(static::$files[$dir][$include_tests][$type])) {
-        $files += static::$files[$dir][$include_tests][$type];
+      if (isset(static::$files[$this->root][$dir][$include_tests][$type])) {
+        $files += static::$files[$this->root][$dir][$include_tests][$type];
       }
     }
 
@@ -478,7 +478,7 @@ protected function scanDirectory($dir, $include_tests) {
       else {
         $filename = $name . '.' . $type;
       }
-      if (!file_exists(dirname($pathname) . '/' . $filename)) {
+      if (!file_exists($this->root . '/' . dirname($pathname) . '/' . $filename)) {
         $filename = NULL;
       }
 
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
new file mode 100644
index 0000000..3ec2e96
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
@@ -0,0 +1,157 @@
+<?php
+
+namespace Drupal\Tests\Core\Extension;
+
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Tests\UnitTestCase;
+use org\bovigo\vfs\vfsStream;
+use Symfony\Component\Yaml\Yaml;
+
+/**
+ * Tests discovery of extensions.
+ *
+ * @coversDefaultClass \Drupal\Core\Extension\ExtensionDiscovery
+ * @group Extension
+ */
+class ExtensionDiscoveryTest extends UnitTestCase {
+
+  /**
+   * Tests extension discovery in a virtual filesystem with vfsStream.
+   *
+   * @covers ::scan
+   */
+  public function testExtensionDiscoveryVfs() {
+
+    // Set up the file system.
+    $filesystem = [];
+    $files_by_type_and_name_expected = $this->populateFilesystemStructure($filesystem);
+
+    $vfs = vfsStream::setup('root', NULL, $filesystem);
+    $root = $vfs->url();
+
+    $this->assertFileExists($root . '/core/modules/system/system.module');
+    $this->assertFileExists($root . '/core/modules/system/system.info.yml');
+
+    // Create an ExtensionDiscovery with $root.
+    $extension_discovery = new ExtensionDiscovery($root, FALSE, NULL, 'sites/default');
+
+    /** @var \Drupal\Core\Extension\Extension[][] $extensions_by_type */
+    $extensions_by_type = [];
+    $files_by_type_and_name = [];
+    foreach (['profile', 'module', 'theme', 'theme_engine'] as $type) {
+      $extensions_by_type[$type] = $extension_discovery->scan($type, FALSE);
+      foreach ($extensions_by_type[$type] as $name => $extension) {
+        $files_by_type_and_name[$type][$name] = $extension->getPathname();
+      }
+      if ($type === 'profile') {
+        // Set profile directories for discovery of the other extension types.
+        $extension_discovery->setProfileDirectories(['myprofile' => 'profiles/myprofile']);
+      }
+    }
+
+    $this->assertEquals($files_by_type_and_name_expected, $files_by_type_and_name);
+
+    $extension_expected = new Extension($root, 'module', 'core/modules/system/system.info.yml', 'system.module');
+    $extension_expected->subpath = 'modules/system';
+    $extension_expected->origin = 'core';
+    $this->assertEquals($extension_expected, $extensions_by_type['module']['system'], 'system');
+
+    $extension_expected = new Extension($root, 'theme_engine', 'core/themes/engines/twig/twig.info.yml', 'twig.engine');
+    $extension_expected->subpath = 'themes/engines/twig';
+    $extension_expected->origin = 'core';
+    $this->assertEquals($extension_expected, $extensions_by_type['theme_engine']['twig'], 'twig');
+  }
+
+  /**
+   * Adds example files to the filesystem structure.
+   *
+   * @param array $filesystem_structure
+   *
+   * @return string[][]
+   *   Format: $[$type][$name] = $yml_file
+   *   E.g. $['module']['system'] = 'system.info.yml'
+   */
+  protected function populateFilesystemStructure(array &$filesystem_structure) {
+    $info_by_file = [
+      'core/profiles/standard/standard.info.yml' => [
+        'type' => 'profile',
+      ],
+      'core/profiles/minimal/minimal.info.yml' => [
+        'type' => 'profile',
+      ],
+      // Override the core instance of the 'minimal' profile.
+      'sites/default/profiles/minimal/minimal.info.yml' => [
+        'type' => 'profile',
+      ],
+      'profiles/myprofile/myprofile.info.yml' => [
+        'type' => 'profile',
+      ],
+      'profiles/myprofile/modules/myprofile_nested_module/myprofile_nested_module.info.yml' => [],
+      'profiles/otherprofile/otherprofile.info.yml' => [
+        'type' => 'profile',
+      ],
+      'core/modules/user/user.info.yml' => [],
+      'profiles/otherprofile/modules/otherprofile_nested_module/otherprofile_nested_module.info.yml' => [],
+      'core/modules/system/system.info.yml' => [],
+      'core/themes/seven/seven.info.yml' => [
+        'type' => 'theme',
+      ],
+      // Override the core instance of the 'seven' theme.
+      'sites/default/themes/seven/seven.info.yml' => [
+        'type' => 'theme',
+      ],
+      'modules/devel/devel.info.yml' => [],
+      'modules/poorly_placed_theme/poorly_placed_theme.info.yml' => [
+        'type' => 'theme',
+      ],
+      'core/themes/engines/twig/twig.info.yml' => [
+        'type' => 'theme_engine',
+      ],
+    ];
+
+    $files_by_type_and_name_expected = [];
+    $content_by_file = [];
+    foreach ($info_by_file as $file => $info) {
+      $name = basename($file, '.info.yml');
+      $info += [
+        'type' => 'module',
+        'name' => "Name of ($name)",
+        'core' => '8.x',
+      ];
+      $type = $info['type'];
+      $content_by_file[$file] = Yaml::dump($info);
+      $files_by_type_and_name_expected[$type][$name] = $file;
+    }
+
+    $content_by_file['core/modules/system/system.module'] = '<?php';
+    $content_by_file['core/themes/engines/twig/twig.engine'] = '<?php';
+
+    foreach ($content_by_file as $file => $content) {
+      $pieces = explode('/', $file);
+      $this->addFileToFilesystemStructure($filesystem_structure, $pieces, $content);
+    }
+
+    unset($files_by_type_and_name_expected['module']['otherprofile_nested_module']);
+
+    return $files_by_type_and_name_expected;
+  }
+
+  /**
+   * @param array $filesystem_structure
+   * @param string[] $pieces
+   *   Fragments of the file path.
+   * @param string $content
+   */
+  protected function addFileToFilesystemStructure(array &$filesystem_structure, array $pieces, $content) {
+    $piece = array_shift($pieces);
+    if ($pieces !== []) {
+      $filesystem_structure += [$piece => []];
+      $this->addFileToFilesystemStructure($filesystem_structure[$piece], $pieces, $content);
+    }
+    else {
+      $filesystem_structure[$piece] = $content;
+    }
+  }
+
+}