strange behavior of TemplateBinding

Question

I am trying to create a custom control. After setting the cornerRadius property to be bound it works only for the default radius which is "8". Here is the definition of property:

public CornerRadius cornerRadius
{
    get { return (CornerRadius)GetValue(cornerRadiusProperty); }
    set { SetValue(cornerRadiusProperty, value); }
}

public static readonly DependencyProperty cornerRadiusProperty =
    DependencyProperty.Register("cornerRadius", typeof(CornerRadius), typeof(expandMenuA), new PropertyMetadata(new CornerRadius(8)));

In the main border, everything works fine, but for the second it works only for default "8" when I change a value in different project, which has a reference to this custom control, nothing happens, and it stays the default:

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:expandMenuA">
    <Style TargetType="{x:Type local:expandMenuA}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:expandMenuA}">
                    <Border CornerRadius="{TemplateBinding cornerRadius}" x:Name="mainBorder" Background="#232323">
                        <StackPanel>
                            <Button Height="{TemplateBinding expanderHeight}" Background="#2d2d2d">
                                <Button.Template>
                                    <ControlTemplate TargetType="Button">
                                        <Border CornerRadius="{TemplateBinding local:expandMenuA.cornerRadius}" Background="{TemplateBinding Background}">
                                            <ContentPresenter HorizontalAlignment="Left" VerticalAlignment="Center"/>
                                        </Border>
                                    </ControlTemplate>
                                </Button.Template>
                                <Button.Content>
                                    <TextBlock Text="{TemplateBinding menuTitle}" Foreground="White" Margin="10, 0, 0, 0" Background="{TemplateBinding BorderBrush}"/>
                                </Button.Content>
                            </Button>
                            <ContentPresenter/>
                        </StackPanel>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    
</ResourceDictionary>

Answer 1

TemplateBinding in ControlTemplate for Button will seek Button's property, which will resolve to default DP value. use Binding with RelativeSource:

<ControlTemplate TargetType="Button">
    <Border CornerRadius="{Binding Path=cornerRadius, RelativeSource={RelativeSource AncestorType={x:Type local:expandMenuA}}}" Background="{TemplateBinding Background}">
        <ContentPresenter HorizontalAlignment="Left" VerticalAlignment="Center"/>
    </Border>
</ControlTemplate>