diff -u b/core/tests/Drupal/FunctionalJavascriptTests/Dialog/DialogPositionTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Dialog/DialogPositionTest.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/Dialog/DialogPositionTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Dialog/DialogPositionTest.php
@@ -104,7 +104,7 @@
 
     $offset_left = (int) str_replace('px', '', $offset_left_evaluated);
 
-    $expected = ($window_width - $dialog_width) / 2;
+    $expected = (($window_width - $dialog_width) / 2) - 1;
 
     $this->assertSame($expected, $offset_left);
   }