What you are facing is an issue colloquially called the Airspace issue. Due to the different underlying graphical engines used in WinForms (GDI) and WPF (DirectX and such), there are some interop issues. Sometimes, your controls appear will be invisible. Sometimes, your WinForms control may overlap your WPF controls.

It is important to know that this is an WPF issue, not a MahApps.Metro one. You can't make the excuse that MahApps.Metro causes this bug but a plain vanilla WPF Window does not. The bug is caused by the Window's AllowsTransparency property.

By default, the vanilla Window class has AllowsTransparency set to false. MahApps.Metro's MetroWindow has it set to true.

One solution to the Airspace issue is to simply set AllowsTransparency to false.

<i:Interaction.Behaviors>
        <Behaviours:BorderlessWindowBehavior AllowsTransparency="False"/>
</i:Interaction.Behaviors>

The second solution, should for some reason that you need AllowsTransparency to be set to true is:

• Go to https://microsoftdwayneneed.codeplex.com/
• Get the lib and add it as a reference to your project.
• Add xmlns:interop="clr-namespace:Microsoft.DwayneNeed.Interop;assembly=Microsoft.DwayneNeed" to your XAML.
• Place your Control in the decorator:

        <interop:AirspaceDecorator AirspaceMode="Redirect"
                                   Background="White"
                                   IsInputRedirectionEnabled="True"
                                   IsOutputRedirectionEnabled="True">
            <!--your winforms control or webbrowser here here-->
        </interop:AirspaceDecorator>

thanks to @myCollections for providing this screenshot from #1038 Chances are, your WebBrowser is being moved by the MetroWindow's built-in TransitioningContentControl.

The solution to the issue is to set the WindowTransitionsEnabled property on your MetroWindow to false.