Border extending too far - wpf

Question

I got this xaml code:
<UserControl x:Class="CharacterMap.CharacterMapControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:CharacterMap"
xmlns:cxml="clr-namespace:CharacterMap.Xml">
<UserControl.Resources>
<ResourceDictionary>
<Style x:Key="Flat">
<Setter Property="Control.Background" Value="White" />
<Setter Property="Control.FontWeight" Value="DemiBold" />
<Setter Property="Control.FontFamily" Value="Arial" />
</Style>
<DataTemplate x:Key="ItemsTemplate">
<Border BorderThickness="0,0,1,1" BorderBrush="Black">
<Button Width="26" Height="23" VerticalContentAlignment="Center" HorizontalContentAlignment="Center" Content="{Binding TheChar}"
Style="{StaticResource Flat}" BorderThickness="0"
ToolTipService.InitialShowDelay="800" ToolTipService.ShowDuration="10000" ToolTipService.BetweenShowDelay="300"
ToolTipService.ShowOnDisabled="True" ToolTip="{Binding AggregatedTooltip}" ToolTipService.IsEnabled="True" />
</Border>
</DataTemplate>
</ResourceDictionary>
</UserControl.Resources>
<Grid >
<Grid.RowDefinitions>
<RowDefinition Height="30"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<StackPanel Orientation="Horizontal" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
<Border BorderBrush="{x:Null}" Height="25" Margin="10,0,20,0">
<TextBlock Text="Filtern nach Buchstabe: " TextWrapping="Wrap" VerticalAlignment="Center"/>
</Border>
<TextBox Text="{Binding FilterString, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="125" Height="25" VerticalContentAlignment="Center" />
</StackPanel>
<ScrollViewer x:Name="ScrollViewer" Margin="10,0,10,10" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Grid.Row="1" >
<Border BorderBrush="Black" BorderThickness="1,1,0,0" SnapsToDevicePixels="True" >
<ItemsControl ItemsSource="{Binding CharDescriptions}" ItemTemplate="{StaticResource ItemsTemplate}" Name="CharItemsControl">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<WrapPanel />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
</ItemsControl>
</Border>
</ScrollViewer>
</Grid>
</UserControl>
It looks something like this:
Before i had the BorderThickness from the button just on 1 but then the borders were overlaying and it looked not so nice.
Now i tried to set the buttons borders thickness to "0,0,1,1" and put a border around the itemscontrol which is "1,1,0,0". The problem is that when i resize the window or in generall the border from the itemscontrol just extends too far and that doesn't look nice either.
How to achieve something like this: ?

Answer 1

You can do something like below. Essentially, you use negative margins to meld all the borders together. On a parent border set a Padding="1" so that the negative margins along the edges can be seen.
The Border around your Button in your DataTemplate would look like this:
<DataTemplate x:Key="ItemsTemplate">
<Border
Margin="-1,-1,0,0"
BorderBrush="Black"
BorderThickness="1">
<Button
Width="26"
Height="23"
HorizontalContentAlignment="Center"
VerticalContentAlignment="Center"
BorderThickness="0"
Content="{Binding TheChar}"
Style="{StaticResource Flat}"
ToolTip="{Binding AggregatedTooltip}"
ToolTipService.BetweenShowDelay="300"
ToolTipService.InitialShowDelay="800"
ToolTipService.IsEnabled="True"
ToolTipService.ShowDuration="10000"
ToolTipService.ShowOnDisabled="True" />
</Border>
</DataTemplate>
The Border around your ItemsControl in your ScrollViewer would have a Padding="1" and look like this:
<ScrollViewer x:Name="ScrollViewer"
Grid.Row="1"
Margin="10,0,10,10"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch">
<Border
Padding="1"
SnapsToDevicePixels="True">
<ItemsControl
Name="CharItemsControl"
ItemTemplate="{StaticResource ItemsTemplate}"
ItemsSource="{Binding CharDescriptions}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<WrapPanel />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
</ItemsControl>
</Border>
</ScrollViewer>