Replace sidebarWindow property with sidebar.iframe.contentWindow

Rather than use a dedicated getter to expose the sidebar frame, it
seemed better on a second look to use the existing `sidebar.iframe`
property that exposes the `<iframe>` element, but document and test that
property.

src/annotator/index.js

@@ -68,7 +68,7 @@ function init() {
     sidebar = new Sidebar(document.body, eventBus, guest, getConfig('sidebar'));
 
     // Expose sidebar window reference for use by same-origin guest frames.
-    window_.__hypothesis.sidebarWindow = sidebar.sidebarWindow;
+    window_.__hypothesis.sidebarWindow = sidebar.iframe.contentWindow;
   }
 
   // Clear `annotations` value from the notebook's config to prevent direct-linked
@@ -77,7 +77,7 @@ function init() {
 
   // Set up communication between this host/guest frame and the sidebar frame.
   const sidebarWindow = sidebar
-    ? sidebar.sidebarWindow
+    ? sidebar.iframe.contentWindow
     : /** @type {HypothesisWindow} */ (window.parent).__hypothesis
         ?.sidebarWindow;
   if (sidebarWindow) {

src/annotator/sidebar.js

@@ -67,7 +67,12 @@ export default class Sidebar {
    */
   constructor(element, eventBus, guest, config = {}) {
     this._emitter = eventBus.createEmitter();
+
+    /**
+     * The `<iframe>` element containing the sidebar application.
+     */
     this.iframe = createSidebarIframe(config);
+
     this.options = config;
 
     /** @type {BucketBar|null} */
@@ -192,13 +197,6 @@ export default class Sidebar {
     this._setupSidebarEvents();
   }
 
-  /**
-   * Return a reference to the `Window` containing the sidebar application.
-   */
-  get sidebarWindow() {
-    return /** @type {Window} */ (this.iframe.contentWindow);
-  }
-
   destroy() {
     this.bucketBar?.destroy();
     this._listeners.removeAll();

src/annotator/test/sidebar-test.js

@@ -145,6 +145,16 @@ describe('Sidebar', () => {
     });
   });
 
+  describe('#iframe', () => {
+    it('returns a reference to the `<iframe>` containing the sidebar', () => {
+      const sidebar = createSidebar();
+      const iframe = containers[0]
+        .querySelector('hypothesis-sidebar')
+        .shadowRoot.querySelector('iframe');
+      assert.equal(sidebar.iframe, iframe);
+    });
+  });
+
   function getConfigString(sidebar) {
     return sidebar.iframe.src;
   }