Request annotation focus after selection in FrameSyncService

If the guest has requested it, request keyboard focus for the first
annotation in the selection after a "showAnnotations" message is
received in FrameSyncService.

src/sidebar/services/frame-sync.js

@@ -366,9 +366,23 @@ export class FrameSyncService {
 
     guestRPC.on(
       'showAnnotations',
-      /** @param {string[]} tags */ tags => {
-        this._store.selectAnnotations(this._store.findIDsForTags(tags));
+      /**
+       * @param {string[]} tags
+       * @param {boolean} focusFirstInSelection - Whether to give keyboard focus
+       *   to the first annotation in the selection.
+       */
+      (tags, focusFirstInSelection = false) => {
+        const ids = this._store.findIDsForTags(tags);
+        this._store.selectAnnotations(ids);
         this._store.selectTab('annotation');
+
+        // Attempt to transfer keyboard focus to the first selected annotation.
+        // This may be blocked in WebKit-based browsers if the user has not
+        // previously interacted with the frame.
+        if (ids.length > 0 && focusFirstInSelection) {
+          this._store.setAnnotationFocusRequest(ids[0]);
+          window.focus();
+        }
       }
     );
 

src/sidebar/services/test/frame-sync-test.js

@@ -138,6 +138,7 @@ describe('FrameSyncService', () => {
         openSidebarPanel: sinon.stub(),
         selectAnnotations: sinon.stub(),
         selectTab: sinon.stub(),
+        setAnnotationFocusRequest: sinon.stub(),
         setSidebarOpened: sinon.stub(),
         toggleSelectedAnnotations: sinon.stub(),
         updateAnchorStatus: sinon.stub(),
@@ -663,6 +664,26 @@ describe('FrameSyncService', () => {
       assert.calledWith(fakeStore.selectAnnotations, ['id1', 'id2', 'id3']);
       assert.calledWith(fakeStore.selectTab, 'annotation');
     });
+
+    it('does not request keyboard focus if `focusFirstInSelection` is false', () => {
+      fakeStore.findIDsForTags.returns(['id1', 'id2', 'id3']);
+      emitGuestEvent(
+        'showAnnotations',
+        ['tag1', 'tag2', 'tag3'],
+        false /* focus */
+      );
+      assert.notCalled(fakeStore.setAnnotationFocusRequest);
+    });
+
+    it('requests keyboard focus for first annotation in selection', () => {
+      fakeStore.findIDsForTags.returns(['id1', 'id2', 'id3']);
+      emitGuestEvent(
+        'showAnnotations',
+        ['tag1', 'tag2', 'tag3'],
+        true /* focus */
+      );
+      assert.calledWith(fakeStore.setAnnotationFocusRequest, 'id1');
+    });
   });
 
   describe('on "hoverAnnotations" message', () => {