JavaScript dialogs cause a page to lose fullscreen.

content/public/browser/web_contents.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_H_
 #define CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 721 matching lines @@
 
   // The callback invoked when the renderer responds to a request for the main
   // frame document's manifest. The url will be empty if the document specifies
   // no manifest, and the manifest will be empty if any other failures occurred.
   typedef base::Callback<void(const GURL&, const Manifest&)>
       GetManifestCallback;
 
   // Requests the manifest URL and the Manifest of the main frame's document.
   virtual void GetManifest(const GetManifestCallback& callback) = 0;
 
+  // Returns whether the renderer is in fullscreen mode.
+  virtual bool IsFullscreenForCurrentTab() const = 0;

[Matt Giuca, 2017/06/09 06:16:37]

Hmmm... It's a bit weird to have the same method in both base classes of
WebContentsImpl. Is there a precedent for this? I guess it's OK.

[Avi (use Gerrit), 2017/06/09 14:36:23]

Yes, this is done in a few places.

+
   // Requests the renderer to exit fullscreen.
   // |will_cause_resize| indicates whether the fullscreen change causes a
   // view resize. e.g. This will be false when going from tab fullscreen to
   // browser fullscreen.
   virtual void ExitFullscreen(bool will_cause_resize) = 0;
 
   // Unblocks requests from renderer for a newly created window. This is
   // used in showCreatedWindow() or sometimes later in cases where
   // delegate->ShouldResumeRequestsForCreatedWindow() indicated the requests
   // should not yet be resumed. Then the client is responsible for calling this
@@ skipping 70 matching lines @@
 
  private:
   // This interface should only be implemented inside content.
   friend class WebContentsImpl;
   WebContents() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_H_