Add example for menus.getTargetElement API (#369)

menu-remove-element/menusGetTargetElementPolyfill.js

@@ -0,0 +1,35 @@
+"use strict";
+
+// The browser.menus.getTargetElement API allows extensions to identify the
+// clicked element that matches a menus.onClicked or menus.onShown event, as of
+// Firefox 63.
+//
+// This polyfill script approximates the behavior, by returning the target of
+// the most recently observed contextmenu event in this document.
+//
+// Limitations:
+// - Cannot return the menu target before this script has run.
+//   In contrast, the real menus.getTargetElement API will return the element
+//   even if the extension did not run any scripts at the time of the click.
+//
+// - Does not offer the guarantee that the element matches the menus.onClicked
+//   or menus.onShown event.
+//   In contrast, the real menus.getTargetElement API only returns an element
+//   if the given first parameter matches the info.targetElementId integer from
+//   the menus.onClicked or menus.onShown events.
+if (!browser.menus || !browser.menus.getTargetElement) {
+  var menuTarget = null;
+  let cleanupIfNeeded = () => {
+    if (menuTarget && !document.contains(menuTarget)) menuTarget = null;
+  };
+  document.addEventListener("contextmenu", (event) => {
+    menuTarget = event.target;
+  }, true);
+  document.addEventListener("visibilitychange", cleanupIfNeeded, true);
+  browser.menus = browser.menus || {};
+  browser.menus.getTargetElement = () => {
+    cleanupIfNeeded();
+    return menuTarget;
+  };
+  true; // Used by popup.js, as variable didUsePolyfill.
+}