ポイントスナップ機能が正しく動作しない

<ControlTemplate x:Key="ResizeDecoratorTemplate" TargetType="{x:Type Control}">
        <Grid>
            <Grid Opacity="0.7" SnapsToDevicePixels="true">
                <!-- 上の回転ツマミ部分 -->
                <control:RotateThumb
                    Width="7"
                    Height="7"
                    Margin="0,-20,0,0"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Top"
                    Cursor="Hand"
                    Template="{StaticResource RotateThumbTemplate}" />
                <!-- 上 -->
                <control:ResizeThumb
                    Height="3"
                    Margin="0,-4,0,0"
                    HorizontalAlignment="Stretch"
                    VerticalAlignment="Top"
                    Cursor="SizeNS"
                    Template="{StaticResource HorizontalResizeHandleTemplate}" />
                <!-- 左 -->
                <control:ResizeThumb
                    Width="3"
                    Margin="-4,0,0,0"
                    HorizontalAlignment="Left"
                    VerticalAlignment="Stretch"
                    Cursor="SizeWE"
                    Template="{StaticResource VerticalResizeHandleTemplate}" />
                <!-- 右 -->
                <control:ResizeThumb
                    Width="3"
                    Margin="0,0,-4,0"
                    HorizontalAlignment="Right"
                    VerticalAlignment="Stretch"
                    Cursor="SizeWE"
                    Template="{StaticResource VerticalResizeHandleTemplate}" />
                <!-- 下 -->
                <control:ResizeThumb
                    Height="3"
                    Margin="0,0,0,-4"
                    HorizontalAlignment="Stretch"
                    VerticalAlignment="Bottom"
                    Cursor="SizeNS"
                    Template="{StaticResource HorizontalResizeHandleTemplate}" />
                <!-- 左上 -->
                <control:ResizeThumb
                    Width="7"
                    Height="7"
                    Margin="-6,-6,0,0"
                    HorizontalAlignment="Left"
                    VerticalAlignment="Top"
                    Cursor="SizeNWSE"
                    Template="{StaticResource ResizeHandleTemplate}" />
                <!-- 右上 -->
                <control:ResizeThumb
                    Width="7"
                    Height="7"
                    Margin="0,-6,-6,0"
                    HorizontalAlignment="Right"
                    VerticalAlignment="Top"
                    Cursor="SizeNESW"
                    Template="{StaticResource ResizeHandleTemplate}" />
                <!-- 左下 -->
                <control:ResizeThumb
                    Width="7"
                    Height="7"
                    Margin="-6,0,0,-6"
                    HorizontalAlignment="Left"
                    VerticalAlignment="Bottom"
                    Cursor="SizeNESW"
                    Template="{StaticResource ResizeHandleTemplate}" />
                <!-- 右下 -->
                <control:ResizeThumb
                    Width="7"
                    Height="7"
                    Margin="0,0,-6,-6"
                    HorizontalAlignment="Right"
                    VerticalAlignment="Bottom"
                    Cursor="SizeNWSE"
                    Template="{StaticResource ResizeHandleTemplate}" />
            </Grid>
            <Grid
                Margin="-3"
                IsHitTestVisible="False"
                Opacity="1">
                <Line
                    Margin="0,-11,0,0"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Top"
                    Stroke="#6c809a"
                    StrokeThickness="1"
                    X1="0"
                    X2="0"
                    Y1="0"
                    Y2="11" />
            </Grid>
        </Grid>
    </ControlTemplate>

public class ToolBarViewModel
    {
        private IDialogService dlgService = null;
        public ObservableCollection<ToolItemData> ToolItems { get; } = new ObservableCollection<ToolItemData>();

        public ToolBarViewModel(IDialogService dialogService)
        {
            this.dlgService = dialogService;
            :
            ToolItems.Add(new ToolItemData("rectangle", "pack://application:,,,/Assets/img/rectangle.png", new DelegateCommand(() =>
            {
                var behavior = new NDrawRectangleBehavior();
                var designerCanvas = App.Current.MainWindow.GetChildOfType<DesignerCanvas>();
                var behaviors = Interaction.GetBehaviors(designerCanvas);
                behaviors.Clear();
                if (!behaviors.Contains(behavior))
                {
                    behaviors.Add(behavior);
                }
                SelectOneToolItem("rectangle");
            })));
            :
        }

        private void SelectOneToolItem(string toolName)
        {
            var toolItem = ToolItems.Where(i => i.Name == toolName).Single();
            toolItem.IsChecked = true;

            ToolItems.Where(i => i.Name != toolName).ToList().ForEach(i => i.IsChecked = false);
        }
    }

internal class NDrawRectangleBehavior : Behavior<DesignerCanvas>
    {
        private Point? _rectangleStartPoint;

        protected override void OnAttached()
        {
            this.AssociatedObject.MouseDown += AssociatedObject_MouseDown;
            this.AssociatedObject.MouseMove += AssociatedObject_MouseMove;
            base.OnAttached();
        }

        protected override void OnDetaching()
        {
            this.AssociatedObject.MouseDown -= AssociatedObject_MouseDown;
            this.AssociatedObject.MouseMove -= AssociatedObject_MouseMove;
            base.OnDetaching();
        }

        private void AssociatedObject_MouseDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                if (e.Source == AssociatedObject)
                {
                    _rectangleStartPoint = e.GetPosition(AssociatedObject);

                    e.Handled = true;
                }
            }
        }

        private void AssociatedObject_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
        {
            var canvas = AssociatedObject as DesignerCanvas;
            if (canvas.SourceConnector == null)
            {
                if (e.LeftButton != MouseButtonState.Pressed)
                    _rectangleStartPoint = null;

                if (_rectangleStartPoint.HasValue)
                {
                    (App.Current.MainWindow.DataContext as MainWindowViewModel).CurrentOperation.Value = "描画";

                    AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(canvas);
                    if (adornerLayer != null)
                    {
                        var adorner = new Adorners.RectangleAdorner(canvas, _rectangleStartPoint);
                        if (adorner != null)
                        {
                            adornerLayer.Add(adorner);
                        }
                    }
                }
            }
        }
    }

internal class RectangleAdorner : Adorner
    {
        private DesignerCanvas _designerCanvas;
        private Point? _startPoint;
        private Point? _endPoint;
        private Pen _rectanglePen;
        private Dictionary<Point, Adorner> _adorners;

        public RectangleAdorner(DesignerCanvas designerCanvas, Point? dragStartPoint)
            : base(designerCanvas)
        {
            _designerCanvas = designerCanvas;
            _startPoint = dragStartPoint;
            var parent = (AdornedElement as DesignerCanvas).DataContext as IDiagramViewModel;
            var brush = new SolidColorBrush(parent.EdgeColors.First());
            brush.Opacity = 0.5;
            _rectanglePen = new Pen(brush, 1);
            _adorners = new Dictionary<Point, Adorner>();
        }

        protected override void OnMouseMove(MouseEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                if (!this.IsMouseCaptured)
                    this.CaptureMouse();

                //ドラッグ終了座標を更新
                _endPoint = e.GetPosition(this);

                var mainWindowVM = (App.Current.MainWindow.DataContext as MainWindowViewModel);
                var designerCanvas = App.Current.MainWindow.GetChildOfType<DesignerCanvas>();

                var diagramVM = mainWindowVM.DiagramViewModel;
                var snapPoints = diagramVM.SnapPoints;
                Point? snapped = null;
                foreach (var snapPoint in snapPoints)
                {
                    if (_endPoint.Value.X > snapPoint.X - mainWindowVM.SnapPower.Value
                     && _endPoint.Value.X < snapPoint.X + mainWindowVM.SnapPower.Value
                     && _endPoint.Value.Y > snapPoint.Y - mainWindowVM.SnapPower.Value
                     && _endPoint.Value.Y < snapPoint.Y + mainWindowVM.SnapPower.Value)
                    {
                        //スナップする座標を一時変数へ保存
                        snapped = snapPoint;
                    }
                }

                //スナップした場合
                if (snapped != null)
                {
                    AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(designerCanvas);
                    RemoveFromAdornerLayerAndDictionary(snapped, adornerLayer);

                    //ドラッグ終了座標を一時変数で上書きしてスナップ
                    _endPoint = snapped;
                    if (adornerLayer != null)
                    {
                        Trace.WriteLine($"Snap={snapped.Value}");
                        if (!_adorners.ContainsKey(snapped.Value))
                        {
                            var adorner = new Adorners.SnapPointAdorner(designerCanvas, snapped.Value);
                            if (adorner != null)
                            {
                                adornerLayer.Add(adorner);

                                //ディクショナリに記憶する
                                _adorners.Add(snapped.Value, adorner);
                            }
                        }
                    }
                }
                else //スナップしなかった場合
                {
                    RemoveAllAdornerFromAdornerLayerAndDictionary(designerCanvas);
                }

                (App.Current.MainWindow.DataContext as MainWindowViewModel).Details.Value = $"({_startPoint.Value.X}, {_startPoint.Value.Y}) - ({_endPoint.Value.X}, {_endPoint.Value.Y}) (w, h) = ({_endPoint.Value.X - _startPoint.Value.X}, {_endPoint.Value.Y - _startPoint.Value.Y})";

                this.InvalidateVisual();
            }
            else
            {
                if (this.IsMouseCaptured) this.ReleaseMouseCapture();
            }

            e.Handled = true;
        }

        private void RemoveAllAdornerFromAdornerLayerAndDictionary(DesignerCanvas designerCanvas)
        {
            AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(designerCanvas);
            var removes = _adorners.ToList();

            removes.ForEach(x =>
            {
                if (adornerLayer != null)
                {
                    adornerLayer.Remove(x.Value);
                }
                _adorners.Remove(x.Key);
            });
        }

        private void RemoveFromAdornerLayerAndDictionary(Point? snapped, AdornerLayer adornerLayer)
        {
            var removes = _adorners.Where(x => x.Key != snapped)
                                                       .ToList();
            removes.ForEach(x =>
            {
                if (adornerLayer != null)
                {
                    adornerLayer.Remove(x.Value);
                }
                _adorners.Remove(x.Key);
            });
        }

        protected override void OnMouseUp(System.Windows.Input.MouseButtonEventArgs e)
        {
            // release mouse capture
            if (this.IsMouseCaptured) this.ReleaseMouseCapture();

            // remove this adorner from adorner layer
            AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(_designerCanvas);
            if (adornerLayer != null)
            {
                adornerLayer.Remove(this);

                foreach (var adorner in _adorners)
                    adornerLayer.Remove(adorner.Value);

                _adorners.Clear();
            }

            if (_startPoint.HasValue && _endPoint.HasValue)
            {
                var rand = new Random();
                var item = new NRectangleViewModel();
                item.Owner = (AdornedElement as DesignerCanvas).DataContext as IDiagramViewModel;
                item.Left.Value = Math.Min(_startPoint.Value.X, _endPoint.Value.X);
                item.Top.Value = Math.Min(_startPoint.Value.Y, _endPoint.Value.Y);
                item.Width.Value = Math.Max(_startPoint.Value.X - _endPoint.Value.X, _endPoint.Value.X - _startPoint.Value.X);
                item.Height.Value = Math.Max(_startPoint.Value.Y - _endPoint.Value.Y, _endPoint.Value.Y - _startPoint.Value.Y);
                item.EdgeColor = item.Owner.EdgeColors.First();
                item.FillColor = item.Owner.FillColors.First();
                item.ZIndex.Value = item.Owner.Items.Count;
                item.IsSelected = true;
                item.Owner.DeselectAll();
                ((AdornedElement as DesignerCanvas).DataContext as IDiagramViewModel).AddItemCommand.Execute(item);

                _startPoint = null;
                _endPoint = null;
            }

            (App.Current.MainWindow.DataContext as MainWindowViewModel).CurrentOperation.Value = "";
            (App.Current.MainWindow.DataContext as MainWindowViewModel).Details.Value = "";

            e.Handled = true;
        }

        protected override void OnRender(DrawingContext dc)
        {
            base.OnRender(dc);

            dc.DrawRectangle(Brushes.Transparent, null, new Rect(RenderSize));

            if (_startPoint.HasValue && _endPoint.HasValue)
                dc.DrawRectangle(Brushes.Transparent, _rectanglePen, new Rect(_startPoint.Value, _endPoint.Value));
        }
    }

public class DiagramViewModel : BindableBase, IDiagramViewModel, IDisposable
    {
        :
        public IEnumerable<Point> SnapPoints
        {
            get {
                var designerCanvas = App.Current.MainWindow.GetChildOfType<DesignerCanvas>();
                return designerCanvas.EnumerateChildOfType<ResizeThumb>()
                      .Where(x => !(x is null))
                      .Select(x => x.TransformToAncestor(designerCanvas).Transform(new Point(0, 0)))
                      .Distinct(new SnapPointDistincter());
            }
        }
        :
    }

class SnapPointDistincter : IEqualityComparer<Point>
    {
        public bool Equals(Point a, Point b)
        {
            var mainWindowVM = (App.Current.MainWindow.DataContext as MainWindowViewModel);
            var x = Math.Abs(a.X - b.X);
            var y = Math.Abs(a.Y - b.Y);
            var r = Math.Sqrt(Math.Pow(x, 2) + Math.Pow(y, 2));
            return r < mainWindowVM.SnapPower.Value / 2;
        }

        public int GetHashCode(Point obj)
        {
            return obj.GetHashCode();
        }
    }