Discovered a strange translation occurring on centered zoom (zoom<>1) after a pan.
The goal is to zoom always on a constant point of the screen, presented by a red circle. Pan is controlled with arrow keys, zoom - with key plus(+) or minus(-).
Everything works fine, except when scaling with zoom <>1 after a pan. Cannot figure out where this one-time translation comes from... Before and after this event panning and zooming work as expected.
Here is the code. What am I missing ?

 <UserControl x:Class="ZoPanNS.ZoPan"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Width="600" Height="600">
    <Canvas Background="White">
        <Ellipse Canvas.Left="200" Canvas.Top="200" Canvas.ZIndex="2"
                Width="12" Height="12" Stroke="Red" StrokeThickness="2" Fill="Transparent" />
        <TextBlock x:Name="TxtB" Canvas.Left="0" Canvas.Top="0" Canvas.ZIndex="2"
                Width="200" Height="20" Text="click here to start" />
        
        <Canvas x:Name="cMoved" Background="PaleGreen" Canvas.Left="100" Canvas.Top="100"
                Width="200" Height="200" RenderTransformOrigin="0,0">
            <Rectangle Canvas.Left="0" Canvas.Top="0"
                Width="200" Height="200" Stroke="DarkCyan" StrokeThickness="2" Fill="Wheat" />
            <Ellipse x:Name="Ccircle" Canvas.Left="100" Canvas.Top="100"
                Width="7" Height="7" Stroke="Green" StrokeThickness="2" Fill="Green" />
        </Canvas>
    </Canvas>
</UserControl>

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;

namespace ZoPanNS
{
    public partial class ZoPan : UserControl
    {
        double Xstep = 40, Ystep = 40, Zstep = 0.2;
        double CenterX=100, CenterY=100;
        private MatrixTransform _matrixTransform = new MatrixTransform();

        public ZoPan()
        {
            InitializeComponent();
            this.Loaded += new RoutedEventHandler(Page_Loaded);
        }
        private void Page_Loaded(object o, EventArgs e)
        {
            cMoved.RenderTransform = _matrixTransform;
            cMoved.RenderTransformOrigin = new Point(.5, .5); //center
            this.KeyDown += new KeyEventHandler(page_KeyDown);
        }
        private void page_KeyDown(object sender, KeyEventArgs e)
        {
            Matrix matrix = _matrixTransform.Matrix;
            switch (e.Key)
            {
                case Key.Left:
                    CenterX += Xstep;
                    Ccircle.SetValue(Canvas.LeftProperty, CenterX);
                    matrix.OffsetX -= Xstep * matrix.M11;
                    break;
                case Key.Right:
                    CenterX -= Xstep;
                    Ccircle.SetValue(Canvas.LeftProperty, CenterX);
                    matrix.OffsetX += Xstep * matrix.M11;
                    break;
                case Key.Up:
                    CenterY += Ystep;
                    Ccircle.SetValue(Canvas.TopProperty, CenterY);
                    matrix.OffsetY -= Ystep * matrix.M22;
                    break;
                case Key.Down:
                    CenterY -= Ystep;
                    Ccircle.SetValue(Canvas.TopProperty, CenterY);
                    matrix.OffsetY += Ystep * matrix.M22;
                    break;

                case Key.Add:
                    matrix.M11 += Zstep;
                    matrix.M22 = matrix.M11;
                    cMoved.RenderTransformOrigin = new Point(
                        CenterX / cMoved.ActualWidth, CenterY / cMoved.ActualHeight);
                    break;

                case Key.Subtract:
                    matrix.M11 -= Zstep;
                    matrix.M22 = matrix.M11;
                    cMoved.RenderTransformOrigin = new Point(
                        CenterX / cMoved.ActualWidth, CenterY / cMoved.ActualHeight);
                    break;
                default:
                    return;
            }
            _matrixTransform.Matrix = matrix;
            TxtB.Text = "zoom=" + matrix.M11.ToString("0.0")
                + " cX=" + CenterX.ToString("0.0")
                + " oX=" + matrix.OffsetX.ToString("0.0")
                + " rX=" + cMoved.RenderTransformOrigin.X.ToString("0.0")
            ;
        }
    }
}