The RadBadge control requires an AdornerLayer, in order to be displayed. When creating a custom ControlTemplate for the Window, you could wrap the root element with an AdornerDecorator instance to apply an AdornerLayer onto its child elements in the visual tree. 

The following code snippet shows a sample implementation of the above logic:

<Window.Template>
    <ControlTemplate TargetType="{x:Type Window}">
        <AdornerDecorator>
            <Grid>
                <ContentPresenter/>
            </Grid>
        </AdornerDecorator>
    </ControlTemplate>
</Window.Template>

Regards,
Stenly
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