Ugly border on Border WPF

Question

I have simple UserControl with image and popup.

<UserControl x:Class="Dziennik.Controls.ImageButton"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="30" d:DesignWidth="100">
    <Button x:Name="button" DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}" Command="{Binding Command}" CommandParameter="{Binding CommandParameter}"
            Background="Transparent"
            BorderBrush="Transparent"
            BorderThickness="0">
        <StackPanel Orientation="Horizontal">
            <Image x:Name="image" Source="{Binding ImageSource}" VerticalAlignment="Center"/>
            <Popup PlacementTarget="{Binding ElementName=button}" Placement="Bottom" IsOpen="{Binding IsMouseOver, ElementName=button,Mode=OneWay}">
                <Border BorderThickness="0" Background="#FFBEE6FD">
                    <TextBlock Text="{Binding Text}" FontWeight="Bold" FontSize="14" Margin="10,5,10,5" VerticalAlignment="Center" HorizontalAlignment="Center"/>
                    </Border>
            </Popup>
        </StackPanel>
    </Button>
</UserControl>

My problem is that I set BorderThickness to 0, but on border sometimes i can see small black border depending on border width.

I will use images to explain my problem.
I have this: https://i.sstatic.net/StVTF.png
Instead of this: https://i.sstatic.net/NuK2H.png

EDIT: SOLUTION

Okay, I've finally found a solution. I had to add AllowsTransparency="True" in Popup properties. Code now looks like this:

<UserControl x:Class="Dziennik.Controls.ImageButton"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="30" d:DesignWidth="100">
    <Button x:Name="button" DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}" Command="{Binding Command}" CommandParameter="{Binding CommandParameter}"
            Background="Transparent"
            BorderBrush="Transparent"
            BorderThickness="0">
        <StackPanel Orientation="Horizontal">
            <Image x:Name="image" Source="{Binding ImageSource}" VerticalAlignment="Center"/>
            <Popup PlacementTarget="{Binding ElementName=button}" Placement="Bottom" IsOpen="{Binding IsMouseOver, ElementName=button,Mode=OneWay}" AllowsTransparency="True">
                <Border BorderThickness="0" Background="#FFBEE6FD">
                    <TextBlock Text="{Binding Text}" FontWeight="Bold" FontSize="14" Margin="10,5,10,5" VerticalAlignment="Center" HorizontalAlignment="Center"/>
                </Border>
            </Popup>
        </StackPanel>
    </Button>
</UserControl>

Answer 1

SOLUTION

Okay, I've finally found a solution. I had to add AllowsTransparency="True" in Popup properties. Code now looks like this:

<UserControl x:Class="Dziennik.Controls.ImageButton"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="30" d:DesignWidth="100">
    <Button x:Name="button" DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}" Command="{Binding Command}" CommandParameter="{Binding CommandParameter}"
            Background="Transparent"
            BorderBrush="Transparent"
            BorderThickness="0">
        <StackPanel Orientation="Horizontal">
            <Image x:Name="image" Source="{Binding ImageSource}" VerticalAlignment="Center"/>
            <Popup PlacementTarget="{Binding ElementName=button}" Placement="Bottom" IsOpen="{Binding IsMouseOver, ElementName=button,Mode=OneWay}" AllowsTransparency="True">
                <Border BorderThickness="0" Background="#FFBEE6FD">
                    <TextBlock Text="{Binding Text}" FontWeight="Bold" FontSize="14" Margin="10,5,10,5" VerticalAlignment="Center" HorizontalAlignment="Center"/>
                </Border>
            </Popup>
        </StackPanel>
    </Button>
</UserControl>