A WebView control in Windows Store app is used to display HTML content such as a URL. If you use a WebView control in your application, you will see your AppBar or Charms are hidden behind the WebView control.

This is because a WebView control is not really a XAML element and does not have same events and rendering process as other XAML controls.

To overcome this problem, Microsoft recommends to use a WebViewBrush.

Here is a good blog on the same:

http://blogs.msdn.com/b/priozersk/archive/2012/08/13/how-to-display-charms-on-a-top-of-the-webview.aspx