In WeekView, when ShowWeekGroupHeaders is set, templating the GroupHeaders with GroupHeaderContentTemplateSelector lead to a rendering issue (see attached files).

In case I forgot something, here is my GroupHeaderContentTemplateSelector definition : 

    <svcore:GroupHeaderContentTemplateSelector x:Key="_GroupHeaderContentTemplateSelectorKey">
      <svcore:GroupHeaderContentTemplateSelector.HorizontalTemplate>
        <DataTemplate>
          <ContentPresenter MinHeight="16" Margin="4" Content="{Binding FormattedName, Converter={StaticResource UpperCaseStringConverter}}"/>
        </DataTemplate>
      </svcore:GroupHeaderContentTemplateSelector.HorizontalTemplate>
      <svcore:GroupHeaderContentTemplateSelector.VerticalTemplate>
        <DataTemplate>
          <ContentPresenter Margin="4" MinHeight="16" VerticalAlignment="Top" Content="{Binding FormattedName, Converter={StaticResource UpperCaseStringConverter}}">
            <ContentPresenter.LayoutTransform>
              <RotateTransform Angle="-90"/>
            </ContentPresenter.LayoutTransform>
          </ContentPresenter>
        </DataTemplate>
      </svcore:GroupHeaderContentTemplateSelector.VerticalTemplate>
      <svcore:GroupHeaderContentTemplateSelector.HorizontalResourceTemplate>
        <DataTemplate>
          <Border Background="{Binding Name.DBResource.Color, Converter={StaticResource _SharedStringColorToSolidColorBrushConverterKey}}" Height="60" Margin="2">
            <StackPanel Orientation="Vertical" Margin="8">
              <TextBlock Foreground="{Binding Name.DBResource.Color, Converter={StaticResource _SharedBackgroundStringColorToForegroundConverterKey}}" FontSize="16" Margin="0" Text="{Binding Name.DisplayName}" />
              <TextBlock Foreground="{Binding Name.DBResource.Color, Converter={StaticResource _SharedBackgroundStringColorToForegroundConverterKey}}" FontSize="12" Text="{Binding Name.DBResource.Code}" />
            </StackPanel>
          </Border>
        </DataTemplate>
      </svcore:GroupHeaderContentTemplateSelector.HorizontalResourceTemplate>
      <svcore:GroupHeaderContentTemplateSelector.VerticalResourceTemplate>
        <DataTemplate>
          <Border Background="{Binding Name.DBResource.Color, Converter={StaticResource _SharedStringColorToSolidColorBrushConverterKey}}" Width="140" Margin="2">
            <StackPanel Margin="8">
              <TextBlock Foreground="{Binding Name.DBResource.Color, Converter={StaticResource _SharedBackgroundStringColorToForegroundConverterKey}}" FontSize="16" Margin="-2 3 0 5"  Text="{Binding Name.DisplayName}" />
              <TextBlock Foreground="{Binding Name.DBResource.Color, Converter={StaticResource _SharedBackgroundStringColorToForegroundConverterKey}}" FontSize="12" Text="{Binding Name.DBResource.Code}" />
            </StackPanel>
          </Border>
        </DataTemplate>
      </svcore:GroupHeaderContentTemplateSelector.VerticalResourceTemplate>
    </svcore:GroupHeaderContentTemplateSelector>