ScrollViewer is not working in WPF WindowsFormHost

Question

I have WindowsFormHost with a RichTextBox in my WPF form, i have given ScrollViewer for

Answer 1

We are using multiple ScrollViewers and also a ViewBox so none of the mentioned solutions worked for us. So here is our soloution which can also deal with different DPI settings.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Forms.Integration;
using System.Windows.Media;
using System.Windows.Threading;


namespace XYZ
{

    public class ClippingWindowsFormsHost : WindowsFormsHost
    {
        private readonly DispatcherTimer _updateTimer;

        private Rect _bounds;

        private PresentationSource _source;

        public ClippingWindowsFormsHost()
        {
            PresentationSource.AddSourceChangedHandler(this, _sourceChangedEventHandler);

            _updateTimer = new DispatcherTimer(DispatcherPriority.Render);
            _updateTimer.Tick += _updateTick;
            _updateTimer.Interval = TimeSpan.FromMilliseconds(100);
        }

        private void _updateTick(object sender, EventArgs e)
        {
            _updateTimer.Stop();

            if (_source == null)
                return;


            // Get the Rect of the scrollviewer on screen.
            Rect scrollRect = _getScrollRect();

            // apply dpi settings
            scrollRect = _scaleDpi(scrollRect);

            if (scrollRect.Width > 0 && scrollRect.Height > 0) // if the rect is valid...
            {
                int x1 = (int) Math.Ceiling(scrollRect.X);
                int y1 = (int) Math.Ceiling(scrollRect.Y);
                int x2 = (int) Math.Ceiling(scrollRect.Right);
                int y2 = (int) Math.Ceiling(scrollRect.Bottom);

                SetWindowRgn(Handle, CreateRectRgn(x1, y1, x2, y2), true);
            }
            else
                SetWindowRgn(Handle, CreateRectRgn(0, 0, 0, 0), true);

        }

        private Rect _scaleDpi(Rect rect)
        {
            if (_source.CompositionTarget != null)
            {
                Matrix transformToDevice = _source.CompositionTarget.TransformToDevice;
                if (!transformToDevice.IsIdentity)
                {
                    Point scaledSize = transformToDevice.Transform(new Point(rect.Width, rect.Height));
                    rect = new Rect(rect.X, rect.Y, scaledSize.X, scaledSize.Y);
                }
            }

            return rect;
        }

        [DllImport("User32.dll", SetLastError = true)]
        private static extern int SetWindowRgn(IntPtr hWnd, IntPtr hRgn, bool bRedraw);

        [DllImport("gdi32.dll")]
        private static extern IntPtr CreateRectRgn(int nLeftRect, int nTopRect, int nRightRect, int nBottomRect);

        protected override void OnWindowPositionChanged(Rect rcBoundingBox)
        {
            base.OnWindowPositionChanged(rcBoundingBox);
            _updateClipping(rcBoundingBox);
        }

        private void _updateClipping(Rect bounds)
        {
            if (_source == null || _bounds == bounds)
                return;

            _bounds = bounds;

            // Only update clipping in certain intervals, otherwise splitpanels can create huge cpu load
            _updateTimer.Stop();
            _updateTimer.Start();
        }

        private Rect _getScrollRect()
        {
            ScrollViewer scrollViewer = _getTopScrollViewer();

            // Get the screenposition of the scrollviewer
            Point topLeft = scrollViewer.PointToScreen(new Point(0, 0));
            Point bottomRight = scrollViewer.PointToScreen(new Point(scrollViewer.ViewportWidth, scrollViewer.ViewportHeight));

            Rect scrollRect = new Rect(topLeft, bottomRight);



            // Get "this" position and use it to offset the scrollrect
            // because that is basically the scrolled distance
            Point myPosition = PointToScreen(new Point());
            scrollRect.Offset(-myPosition.X, -myPosition.Y);

            return scrollRect;
        }

        private ScrollViewer _getTopScrollViewer()
        {
            DependencyObject parent = this;
            ScrollViewer lastViewer = null;
            while ((parent = VisualTreeHelper.GetParent(parent)) != null)
            {
                ScrollViewer viewer = parent as ScrollViewer;
                if (viewer != null)
                    lastViewer = viewer;
            }

            return lastViewer;
        }

        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);

            if (disposing)
            {
                _updateTimer.Stop();
                PresentationSource.RemoveSourceChangedHandler(this, _sourceChangedEventHandler);
            }
        }

        private void _sourceChangedEventHandler(object sender, SourceChangedEventArgs e)
        {
            _updateTimer.Stop();
            _source = e.NewSource;
        }
    }
}