Canvas in ScrollViewer (Preview)MouseButtonDown event order
<ScrollViewer Name="scroll_viewer" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
    <Canvas Name="canvas" Height="200" Width="200">
        <Rectangle Fill="AliceBlue" Width="100" Height="100"/>
    </Canvas>
</ScrollViewer>

scroll_viewer.PreviewMouseLeftButtonDown
scroll_viewer.MouseLeftButtonDown
canvas.PreviewMouseLeftButtonDown

((FrameworkElement)scroll_viewer.GetValue(ScrollViewer.ContentProperty)).MouseLeftButtonDown += scroll_viewer_MouseLeftButtonDown;

protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e) {
    if (base.Focus())
        e.Handled = true;
    base.OnMouseLeftButtonDown(e);
}

public class MyScrollViewer : ScrollViewer {

    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e) {
        base.OnMouseLeftButtonDown(e);
        e.Handled = false;
    }
}

this.scroll_viewer.MouseLeftButtonDown += new MouseButtonEventHandler(scroll_viewer_MouseLeftButtonDown);

this.scroll_viewer.AddHandler(ScrollViewer.MouseLeftButtonDownEvent, new MouseButtonEventHandler(this.scroll_viewer_MouseLeftButtonDown), true);