diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index ebe5958..627e4fe 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -70,13 +70,6 @@ class ExtensionDiscovery {
   protected $infoParser;
 
   /**
-   * Static root path, useful for invalidating static::$files when needed.
-   *
-   * @var string
-   */
-  protected static $cacheRoot;
-
-  /**
    * Previously discovered files keyed by origin directory and extension type.
    *
    * @var array
@@ -128,12 +121,6 @@ public function __construct($root, $use_file_cache = TRUE, $profile_directories
     $this->fileCache = $use_file_cache ? FileCacheFactory::get('extension_discovery') : NULL;
     $this->profileDirectories = $profile_directories;
     $this->sitePath = $site_path;
-    // Invalidate the cache if we're using a new root directory. We must do this
-    // since $files is static.
-    if ($this->root != static::$cacheRoot) {
-      static::$files = [];
-      static::$cacheRoot = $this->root;
-    }
   }
 
   /**
@@ -228,12 +215,12 @@ 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];
       }
     }