diff --git a/core/modules/system/tests/src/FunctionalJavascript/OffCanvasTestBase.php b/core/modules/system/tests/src/FunctionalJavascript/OffCanvasTestBase.php
index 12c3d86717..6aa810568d 100644
--- a/core/modules/system/tests/src/FunctionalJavascript/OffCanvasTestBase.php
+++ b/core/modules/system/tests/src/FunctionalJavascript/OffCanvasTestBase.php
@@ -41,6 +41,9 @@ protected function enableTheme($theme) {
    */
   protected function waitForOffCanvasToOpen() {
     $web_assert = $this->assertSession();
+    // Wait just slightly longer than the tray CSS animation.
+    // @see core/misc/dialog/off-canvas.motion.css
+    $this->getSession()->wait(800);
     $web_assert->assertWaitOnAjaxRequest();
     $this->assertElementVisibleAfterWait('css', '#drupal-off-canvas');
   }