When merging the ReportViewer's WPF theme dictionaries from the DLL in App.xaml, the VS2019 Designer fails due to a problem with a ZoomComboBoxStyle definition.

This causes all Telerik UI for WPF control's styles fail to load in the Visual Studio designer.

WORKAROUND

Instead of App.xaml, merge the ReportViewer's theme dictionary closer to the control.

For example in the ReportViewer's direct parent:

<Grid>
<Grid.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<!-- Moved from app.xaml -->
<ResourceDictionary Source="/Telerik.ReportViewer.Wpf.Themes;component/Themes/Summer/Telerik.ReportViewer.Wpf.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Grid.Resources>

<tr:ReportViewer x:Name="ReportViewer1" ReportSource="{Binding CurrentReport}" />
</Grid>