I have a grid with a menu and a scrollviewer inside. In the scrollviewer i have a groupbox with another grid and inside a webbrowser element.
Now the problem arises
The WPF WebBrowser control is basically the old Win32 Webbrowser control and is technically rendered above the WPF content. Additionally it has some serious issues with size calculations.
See http://msdn.microsoft.com/en-us/library/aa970688(v=vs.85).aspx
I'm afraid there's no way to tell the WebBrowser Control to change that behavior.
But there are alternative controls out there:
http://wpfchromium.codeplex.com/
http://awesomium.com/
Edit:
Found a great summary here on SO which points to some alternatives:
WPF WebBrowser (3.5 SP1) Always on top - other suggestion to display HTML in WPF