WPF Overlaying a Image with a Button

Answer 1

that behaviour is because of the DockPanel here:

<DockPanel x:Name="ContentPanel">
        <Button x:Name="PART_Close" Panel.ZIndex="1" HorizontalAlignment="Right" Margin="0,1,1,0" VerticalAlignment="Top" Width="16" Height="16" DockPanel.Dock="Right" Style="{DynamicResource CloseableTabItemButtonStyle}" ToolTip="Close Tab">
          <Path x:Name="Path" Stretch="Fill" StrokeThickness="0.5" Stroke="#FF333333" Fill="#FF969696" Data="F1 M 2.28484e-007,1.33331L 1.33333,0L 4.00001,2.66669L 6.66667,6.10352e-005L 8,1.33331L 5.33334,4L 8,6.66669L 6.66667,8L 4,5.33331L 1.33333,8L 1.086e-007,6.66669L 2.66667,4L 2.28484e-007,1.33331 Z " HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
        </Button>
        <ContentPresenter Panel.ZIndex="0" x:Name="Content" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" ContentSource="Header" RecognizesAccessKey="True" HorizontalAlignment="Center" VerticalAlignment="Center" Margin="{TemplateBinding Padding}"/>
      </DockPanel>

remove the dock panel and adjust your properties as necessary and the problem will be fixed.