Add initial versions

inert.js

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+/**
+@license
+Copyright (c) 2022 trading_peter
+This program is available under Apache License Version 2.0
+*/
+
+/**
+ * This helper provides sort of a inert polyfill.
+ * The `inert` property and a helper method is added that can be used
+ * to update tabindex for example. The implementing element also is styled with
+ * pointer-events: none if the inert property is set true.
+ */
+export const Inert = function(superClass) {
+  return class extends superClass {
+    static get properties() {
+      return {
+        inert: {
+          type: Boolean,
+          value: false,
+          observer: '_inertChanged',
+          reflectToAttribute: true
+        }
+      };
+    }
+
+    updated(changes) {
+      if (changes.has('inert')) {
+        this._inertChanged(this.inert);
+      }
+    }
+
+    _inertChanged(state) {
+      if (state) {
+        this.style.pointerEvents = 'none';
+        this._inertTapIndex = this.getAttribute('tabindex');
+      } else {
+        this.style.pointerEvents = '';
+        if (this._inertTapIndex) {
+          this.setAttribute('tabindex', this._inertTapIndex);
+        }
+      }
+    }
+  };
+}