拖动移动网格不向上移动

Question

我试图用行为拖动画布，我让它适合水平移动，但我不能让它与垂直移动一起工作。当我尝试垂直拖动它时，当我向上移动鼠标时它会跳起来，当我向下移动鼠标时它也会跳起来。它以很大的增量向上移动。水平运动虽然很好，但代码也是一样的。

以下是我的行为代码。

 public class MouseDragPopupBehavior : Behavior<Grid>
{
    private bool _mouseDown;
    private Point _oldMousePosition;

    /// <summary>
    /// Called after the behavior is attached to an AssociatedObject.
    /// </summary>
    /// <remarks>
    /// Override this to hook up functionality to the AssociatedObject.
    /// </remarks>
    protected override void OnAttached()
    {
        AssociatedObject.MouseLeftButtonDown += (sender, e) =>
            {
                _mouseDown = true;
                _oldMousePosition = AssociatedObject.PointToScreen(e.GetPosition(AssociatedObject));
                AssociatedObject.CaptureMouse();
            };

        AssociatedObject.MouseMove += (sender, e) =>
            {
                if (!_mouseDown) return;
                Point newMousePosition = AssociatedObject.PointToScreen(e.GetPosition(AssociatedObject));
                Vector offset = newMousePosition - _oldMousePosition;

                _oldMousePosition = newMousePosition;

                double horizontalMovement = Canvas.GetLeft(AssociatedObject) + offset.X;
                double verticalMovement = Canvas.GetTop(AssociatedObject) + offset.Y;

                // Works
                //double marginShift = AssociatedObject.Margin.Top + offset.Y;
                //AssociatedObject.Margin = new Thickness(0, marginShift, 0, 0);

                Canvas.SetLeft(AssociatedObject, horizontalMovement);

                // Does not work
                Canvas.SetTop(AssociatedObject, verticalMovement);
            };

        AssociatedObject.MouseLeftButtonUp += (sender, e) =>
            {
                _mouseDown = false;
                AssociatedObject.ReleaseMouseCapture();
            };
    }
}

我的代码适用于水平（从左到右）移动，但不适用于垂直移动。我确实通过设置边距来获得垂直移动，如注释部分所示，但我真的想让它以其他方式工作。

感谢您的帮助。