Why is a Line shape on a Canvas drifting away while dragging?

Question

I see a strange behaviour dragging in Line shape over a Canvas that I cannot explain. The Line shape is in a Thumb DataTemplate like so:

Answer 1

I had the same problem and after this answer "For a Line the correct value to animate the dragging of the ellipse on the canvas is the difference of the displacement between two consecutive DragDelta events." I developed this solution, worked for me, hope this helps someone

 public class Connector : Thumb
{
    private double _oldX = 0;
    private double _oldY = 0;


    public Connector()
    {
        DragDelta += Connector_DragDelta;
        DragCompleted += Connector_DragCompleted;
    }

    private void Connector_DragCompleted(object sender, DragCompletedEventArgs e)
    {
        _oldX = 0;
        _oldY = 0;
    }

    private void Connector_DragDelta(object sender, DragDeltaEventArgs e)
    {
        Thumb thumb = e.Source as Thumb;
        DiagramConnectorViewModel viewModel = (DiagramConnectorViewModel)thumb.DataContext;

        double leftOffSet = _oldX - e.HorizontalChange;
        _oldX = e.HorizontalChange;
        viewModel.X1 -= leftOffSet;
        viewModel.X2 -= leftOffSet;

        double topOffSet = _oldY - e.VerticalChange;
        _oldY = e.VerticalChange;
        viewModel.Y1 -= topOffSet;
        viewModel.Y2 -= topOffSet;

    }