JavaScript dialogs cause a page to lose fullscreen.

content/public/browser/web_contents.h

Index: content/public/browser/web_contents.h
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index 3e1ed690eb54a397b4dc557f18de0e0b39156b15..2e46bb250c98852f9cfe0d16e8d884273d10fb9f 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -739,6 +739,9 @@ class WebContents : public PageNavigator,
   // 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;
+
   // 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