This is a known issue in WPF popups
. As the autohide area in RadDocking is a WPF popup in order to show the WebBrowser control you will need to change the AllowsTransparency to false in the theme you are using. The easiest way of achieving this is by using Implicit Styles
If you are using Implicit Styles, after merging the needed dictionaries you will need to open the Telerik.Windows.Controls.Docking.xaml file and change the AllowsTransparency property in all of the popups to false.
We created and attached a sample project for you to take a look at, note that by changing the AllowsTransparency property a black background behind the AutoHide area will appear, which cannot be covered unless the AllowsTransparency is set to true.
Hope this helps.
the Telerik team