WinformsHost inside a ScrollViewer

/// <summary>Top left x position of window region</summary>
private Int32 _topLeftX = -1;

/// <summary>Top left y position of window region</summary>
private Int32 _topLeftY = -1;

/// <summary>Bottom right x position of window region</summary>
private Int32 _bottomRightX = -1;

/// <summary>Bottom right x position of window region</summary>
private Int32 _bottomRightY = -1;

// ...

/// <summary>Creates a handle to a region specified through upper left and bottom right corner.</summary>
/// <param name="x1">Upper left X</param>
/// <param name="y1">Upper left Y</param>
/// <param name="x2">Bottom right X</param>
/// <param name="y2">Bottom right Y</param>
/// <returns>Returns a handle to the created region.</returns>
[DllImport("GDI32.DLL", EntryPoint = "CreateRectRgn")]
private static extern IntPtr CreateRectRgn(Int32 x1, Int32 y1, Int32 x2, Int32 y2);

/// <summary>Sets the specified region to the specified window. That means the specified window can paint
/// itself only in the specified region.</summary>
/// <param name="hWnd">Handle to the window</param>
/// <param name="hRgn">Hanlde to the region</param>
/// <param name="bRedraw">Boolean indicating whether the specified region should be redrawn.</param>
/// <returns>Returns nonzero if success, otherwise zero.</returns>
[DllImport("User32.dll", SetLastError = true)]
private static extern Int32 SetWindowRgn(IntPtr hWnd, IntPtr hRgn, Boolean bRedraw);

/// <summary>
/// Gets the handle of the parent
/// </summary>
/// <param name="hWnd">The Childs hWnd</param>
/// <returns></returns>
[DllImport("User32.dll")]
private static extern IntPtr GetParent(IntPtr hWnd);

private void OnScrollChanged(object sender, ScrollChangedEventArgs scrollChangedEventArgs)
{
    PresentationSource presentationSource = PresentationSource.FromVisual(this);

    Visual rootVisual = presentationSource?.RootVisual;
    if (rootVisual == null)
    {
        return;
    }

    var scrollViewer = sender as ScrollViewer;
    if(scrollViewer == null || !scrollViewer.IsDescendantOf(rootVisual)) return;

    GeneralTransform transform = scrollViewer.TransformToAncestor(rootVisual);
    Rect scrollRect = transform.TransformBounds(new Rect(0, 0, scrollViewer.ViewportWidth, scrollViewer.ViewportHeight));

    // calculate the rect of the scrollable windows forms host instance with 0/0 at upper left corner of root visual
    transform = TransformToAncestor(rootVisual);
    Rect hostRect = transform.TransformBounds(new Rect(Padding.Left, Padding.Right,
      RenderSize.Width, RenderSize.Height));

    // calculate the intersection of the two rect
    Rect intersectRect = Rect.Intersect(scrollRect, hostRect);

    Int32 topLeftX = 0;
    Int32 topLeftY = 0;
    Int32 bottomRightX = 0;
    Int32 bottomRightY = 0;

    if (intersectRect != Rect.Empty)
    {
        // calculate the HRGN points with 0/0 at upper left corner of scrollable windows forms host instance
        topLeftX = (Int32)(intersectRect.TopLeft.X - hostRect.TopLeft.X);
        topLeftY = (Int32)(intersectRect.TopLeft.Y - hostRect.TopLeft.Y);
        bottomRightX = (Int32)(intersectRect.BottomRight.X - hostRect.TopLeft.X);
        bottomRightY = (Int32)(intersectRect.BottomRight.Y - hostRect.TopLeft.Y);
    }

    // because the CreateRectRgn / SetWindowRgn api calls are slow we call them only if it has a visual effect
    if (_topLeftX != topLeftX || _topLeftY != topLeftY || _bottomRightX != bottomRightX || _bottomRightY != bottomRightY)
    {
        _topLeftX = topLeftX;
        _topLeftY = topLeftY;
        _bottomRightX = bottomRightX;
        _bottomRightY = bottomRightY;
        // create HRGN object and set it to the windows forms host instance
        IntPtr hrgn = CreateRectRgn(_topLeftX, _topLeftY, _bottomRightX, _bottomRightY);
        SetWindowRgn(GetParent(_webView.Handle), hrgn, true);
    }
}