return true, cause DocumentFragment is never displayed.

(cherry picked from commit ec9c99554ace24b5a1fbeae4a22dbe29cdc62f77)

src/Umbraco.Web.UI.Client/src/common/services/tabbable.service.js

@@ -186,7 +186,7 @@
 
               if (!nodeComputedStyle) {
                 if (node instanceof DocumentFragment) {
-                  return false;
+                  return true;// though DocumentFragment dosnt directly have display 'none', we know that it will never be visible, and therefor we return true. (and do not cache this, cause it will change if appended to the DOM)
                 } else {
                   nodeComputedStyle = this.doc.defaultView.getComputedStyle(node);
                 }