diff -u b/core/modules/system/src/Tests/Theme/TwigEnvironmentTest.php b/core/modules/system/src/Tests/Theme/TwigEnvironmentTest.php
--- b/core/modules/system/src/Tests/Theme/TwigEnvironmentTest.php
+++ b/core/modules/system/src/Tests/Theme/TwigEnvironmentTest.php
@@ -66,7 +66,8 @@
     $this->assertEqual($renderer->renderRoot($element), 'test-with-context muuh');
     $this->assertEqual($renderer->renderRoot($element_copy), 'test-with-context muuh');
 
-    // Tests caching of inline templates.
+    // Tests caching of inline templates with long content to ensure the
+    // generated cache key can be used as a filename.
     $element = [];
     $element['test'] = [
       '#type' => 'inline_template',