Code

<ScrollViewer
	PreviewMouseWheel="OnMouseWheel" ...

private void OnMouseWheel(object sender, MouseWheelEventArgs e)
{
	Debug.WriteLine(e.GetPosition(diagram.DocumentPlane));
	e.Handled = true;
} 

Code

private void OnCtrMouseWheel(object sender, MouseWheelEventArgs e)
{
	var docPos = e.GetPosition(diagram.DocumentPlane);
	diagram.SetZoomFactor(
		Math.Max(5, diagram.ZoomFactor + e.Delta / 50.0),
		docPos);
	scrollViewer.UpdateLayout();

	var viewerDocBounds = diagram.Viewport;
	if (!diagram.Bounds.Contains(viewerDocBounds))
	{
		diagram.Bounds = Rect.Union(viewerDocBounds, diagram.Bounds);
		scrollViewer.UpdateLayout();
	}
	var newDocPos = e.GetPosition(diagram.DocumentPlane);
	var newScrollX = diagram.ScrollX + docPos.X - newDocPos.X;
	var newScrollY = diagram.ScrollY + docPos.Y - newDocPos.Y;
	if (newScrollX < diagram.Bounds.Left || newScrollY < diagram.Bounds.Top)
	{
		diagram.Bounds = Rect.Union(
			diagram.Bounds, new Point(newScrollX, newScrollY));
		diagram.ScrollX = newScrollX;
		diagram.ScrollY = newScrollY;
	}

	e.Handled = true;
}