Update to Drupal 8.2.4. For more information, see https://www.drupal.org/project/drupal/releases/8.2.4

core/modules/outside_in/js/offcanvas.js

@@ -120,6 +120,9 @@
           of: window
         };
         settings.dialogClass = 'ui-dialog-offcanvas';
+        // Applies initial height to dialog based on window height.
+        // See http://api.jqueryui.com/dialog for all dialog options.
+        settings.height = $(window).height();
       }
     },
     'dialog:beforeclose': function (event, dialog, $element) {