Fix3687 - card border white edges (#3691)

* add card to demo app showcasing this bug

* removed Background of nested Border in the AdornerLayer

* added Margin and CornerRadius to the inner Border of the AdornerDecorator in the Card, to circumvent white edges showing up around the Card

Author: corvinsz

src/MainDemo.Wpf/Cards.xaml

@@ -24,6 +24,22 @@
     <TextBlock Style="{StaticResource PageTitleTextBlock}" Text="Card" />
 
     <WrapPanel>
+
+      <smtx:XamlDisplay Margin="0,0,8,8" UniqueKey="cards_11111">
+        <Grid Background="Blue">
+          <materialDesign:Card materialDesign:ElevationAssist.Elevation="Dp24" Margin="25" UniformCornerRadius="100" Padding="0" BorderThickness="0" >
+            <Grid Background="Black" Height="300" Width="300" Margin="0">
+              <TextBlock HorizontalAlignment="Center"
+                         VerticalAlignment="Center"
+                         Foreground="White"
+                         Text="The quick brown fox jumps over the lazy dog"
+                         Margin="20"
+                         TextWrapping="Wrap"/>
+            </Grid>
+          </materialDesign:Card>
+        </Grid>
+      </smtx:XamlDisplay>
+
       <smtx:XamlDisplay Margin="0,0,8,8" UniqueKey="cards_1">
         <materialDesign:Card Width="200">
           <Grid>

src/MainDemo.Wpf/Properties/launchSettings.json

@@ -2,7 +2,7 @@
     "profiles": {
         "Demo App": {
             "commandName": "Project",
-            "commandLineArgs": "-p Home -t Inherit -f LeftToRight"
+            "commandLineArgs": "-p Card -t Light -f LeftToRight"
         }
     }
 }

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Card.xaml

@@ -34,6 +34,8 @@
                 >
                 <Border Padding="{TemplateBinding Padding}"
                         Background="{TemplateBinding Background}"
+                        Margin="1"
+                        CornerRadius="{TemplateBinding UniformCornerRadius, Converter={x:Static converters:DoubleToCornerRadiusConverter.Instance}}"
                         Clip="{TemplateBinding ContentClip}"/>
               </Border>
             </AdornerDecorator>