PDF Viewer: Restore focus after exiting Presentation Mode.

This is necessary so that up/down arrow keys trigger scrolling of
the PDF document after exiting Presentation mode.

Bug: 1148478
Change-Id: Ie64d1e3deb5763a43ef4dc7f094065992b5eb4c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597770
Commit-Queue: dpapad <dpapad@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840131}

chrome/browser/resources/pdf/pdf_viewer.js

@@ -735,6 +735,9 @@ export class PDFViewerElement extends PDFViewerBaseElement {
             assert(document.fullscreenElement === null);
             scroller.removeEventListener('wheel', onWheel);
             this.pluginController_.setReadOnly(false);
+
+            // Ensure that directional keys still work after exiting.
+            this.shadowRoot.querySelector('embed').focus();
           });
 
           // Nothing else to do here. The viewport will be updated as a result

chrome/test/data/pdf/BUILD.gn

@@ -137,6 +137,7 @@ js_library("fullscreen_test") {
     "//chrome/browser/resources/pdf:pdf_viewer",
     "//third_party/polymer/v3_0/components-chromium/iron-test-helpers:mock-interactions",
     "//ui/webui/resources/js:cr.m",
+    "//ui/webui/resources/js:util.m",
   ]
   externs_list = [ "$externs_path/test.js" ]
 }

chrome/test/data/pdf/fullscreen_test.js

@@ -7,6 +7,7 @@ import {FittingType} from 'chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/c
 import {PDFScriptingAPI} from 'chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/pdf_scripting_api.js';
 import {PDFViewerElement} from 'chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/pdf_viewer.js';
 import {isMac} from 'chrome://resources/js/cr.m.js';
+import {getDeepActiveElement} from 'chrome://resources/js/util.m.js';
 import {keyDownOn} from 'chrome://resources/polymer/v3_0/iron-test-helpers/mock-interactions.js';
 
 import {createWheelEvent} from './test_util.js';
@@ -140,6 +141,16 @@ const tests = [
       chrome.test.succeed();
     });
   },
+  // Note: The following test needs to be the last one, because subsequent calls
+  // to requestFullScreen() fail with an "API can only be initiated by a user
+  // gesture" error.
+  async function testFocusAfterExiting() {
+    await ensureFullscreen();
+    document.exitFullscreen();
+    await eventToPromise('fullscreenchange', scroller);
+    chrome.test.assertEq('EMBED', getDeepActiveElement().nodeName);
+    chrome.test.succeed();
+  },
 ];
 
 chrome.test.runTests(tests);