I have reduced the picture to a point where it cannot be reduced anymore. Double-clicking on any point of the picture does not move properly to the middle of the view. What changes do I need to make?

private void diagram_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
        {
            var zoomRect = diagram.Viewport;
            if (e.Delta < 0)
                zoomRect.Inflate(100, 100);
            else
                zoomRect.Inflate(-100, -100);
            if (diagram.Bounds.Width >= zoomRect.Width && diagram.Bounds.Height >= zoomRect.Height)
                diagram.ZoomToRect(zoomRect, true);

            // stop the scrollviewer from scrolling
            e.Handled = true;
        }

private void diagram_NodeDoubleClicked(object sender, NodeEventArgs e)
        {
            var node = e.Node;

            if (node.MasterGroup != null)
                node = node.MasterGroup.MainItem as DiagramNode;

            var posX = e.MousePosition.X - (diagram.Viewport.X + diagram.Viewport.Width / 2);
            var posY = e.MousePosition.Y - (diagram.Viewport.Y + diagram.Viewport.Height / 2);

            diagram.ScrollX += posX;
            diagram.ScrollY += posY;
        }

Thank you, Lyubo, I see