refactor: add private property for detecting mixin

Author: ugur-vaadin

packages/dashboard/src/vaadin-dashboard-helpers.js

@@ -144,9 +144,5 @@ export function findAncestorInstance(node, baseClass) {
  * @returns {HTMLElement | null}
  */
 export function getParentLayout(node) {
-  return __findFilteredAncestorInstance(node, (el) => {
-    return (
-      el.constructor && (el.constructor.is === 'vaadin-dashboard' || el.constructor.is === 'vaadin-dashboard-layout')
-    );
-  });
+  return __findFilteredAncestorInstance(node, (el) => el.__hasVaadinDashboardLayoutMixin);
 }

packages/dashboard/src/vaadin-dashboard-layout-mixin.js

@@ -20,6 +20,16 @@ export const DashboardLayoutMixin = (superClass) =>
   class DashboardLayoutMixinClass extends ResizeMixin(superClass) {
     static get properties() {
       return {
+        /**
+         * Used for mixin detection because `instanceof` does not work with mixins.
+         *
+         * @private
+         * @type {boolean}
+         */
+        __hasVaadinDashboardLayoutMixin: {
+          value: true,
+        },
+
         /**
          * Whether the dashboard layout is dense.
          *