fixed a bug of CoverView

HandyControl/Controls/Cover/CoverView/CoverView.cs

@@ -182,6 +182,7 @@ namespace HandyControl.Controls
                 _viewContent.ManualHeight = 0;
                 if (_viewContent.Content is FrameworkElement element)
                 {
+                    element.VerticalAlignment = VerticalAlignment.Top;
                     element.Arrange(new Rect(new Size(double.MaxValue, double.MaxValue)));
                     _viewContent.ManualHeight = element.ActualHeight;
                 }

HandyControl/Themes/Styles/Base/CoverViewBaseStyle.xaml

@@ -38,7 +38,7 @@
                                 </TransformGroup>
                             </Border.RenderTransform>
                         </Border>
-                        <Border x:Name="PART_Content" Height="0" Grid.Row="1" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}">
+                        <Border x:Name="PART_Content" MinHeight="30" Height="0" Grid.Row="1" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}">
                             <ContentPresenter Margin="{TemplateBinding Padding}"/>
                         </Border>
                         <Rectangle SnapsToDevicePixels="True" Visibility="{Binding Visibility,ElementName=PART_Triangle}" Grid.Row="1" VerticalAlignment="Top" HorizontalAlignment="Left" Width="28" Margin="{Binding Margin,ElementName=PART_Triangle}" Height="1" Fill="{TemplateBinding Background}">