ScrollViewer is not working in WPF WindowsFormHost

Finally got the solution

Create this Class in your solution for above problem, and take new class control (ScrollViewerWindowsFormsHost) instead of WindowsFormsHost

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Windows.Forms.Integration;
using System.Windows.Media;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Controls;

namespace WPFRichTextBox
{
class ScrollViewerWindowsFormsHost: WindowsFormsHost
{

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

        if (ParentScrollViewer == null)
            return;

        GeneralTransform tr = ParentScrollViewer.TransformToAncestor(MainWindow);
        var scrollRect = new Rect(new Size(ParentScrollViewer.ViewportWidth, ParentScrollViewer.ViewportHeight));
        scrollRect = tr.TransformBounds(scrollRect);

        var intersect = Rect.Intersect(scrollRect, rcBoundingBox);
        if (!intersect.IsEmpty)
        {
            tr = MainWindow.TransformToDescendant(this);
            intersect = tr.TransformBounds(intersect);
        }

        SetRegion(intersect);
    }

    protected override void OnVisualParentChanged(DependencyObject oldParent)
    {
        base.OnVisualParentChanged(oldParent);
        ParentScrollViewer = null;

        var p = Parent as FrameworkElement;
        while (p != null)
        {
            if (p is ScrollViewer)
            {
                ParentScrollViewer = (ScrollViewer)p;
                break;
            }

            p = p.Parent as FrameworkElement;
        }
    }

    private void SetRegion(Rect intersect)
    {
        using (var graphics = System.Drawing.Graphics.FromHwnd(Handle))
            SetWindowRgn(Handle, (new System.Drawing.Region(ConvertRect(intersect))).GetHrgn(graphics), true);
    }

    static System.Drawing.RectangleF ConvertRect(Rect r)
    {
        return new System.Drawing.RectangleF((float)r.X, (float)r.Y, (float)r.Width, (float)r.Height);
    }

    private Window _mainWindow;
    Window MainWindow
    {
        get
        {
            if (_mainWindow == null)
                _mainWindow = Window.GetWindow(this);

            return _mainWindow;
        }
    }

    ScrollViewer ParentScrollViewer { get; set; }

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

}

XAML Code:

<Window x:Class="WPFRichTextBox.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:wf="clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms"
    xmlns:swfh="clr-namespace:WPFRichTextBox"
    Title="MainWindow" Height="600" Width="800" Background="LightBlue">
<Grid Loaded="Grid_Loaded">

    <ScrollViewer  Background="DarkOrange" VerticalScrollBarVisibility="Auto" Height="100"  Margin="11,160,12,301" Width="756" Name="scrollViewer1">
        <Canvas Height="200" Name="canvas1" Width="auto" >
      <swfh:ScrollableWindowsFormsHost ClipToBounds="True" Height="194" Width="715" Margin="10,5,0,0" Background="Gray">
                <wf:RichTextBox BackColor="Cornsilk" Text="RichTextBox" x:Name="richTbTest" BorderStyle="None" Enabled="True" ForeColor="Black" Width="550" Multiline="True" ReadOnly="True" />
      </swfh:ScrollableWindowsFormsHost>
        </Canvas>
    </ScrollViewer>
</Grid>

Just in case someone else has my edge case, where I have a WinForms UserControl hosted inside a WPF UserControl, which itself is hosted inside a WinForms Form (don't ask...) - the class Avinash provided didn't fix my clipping issues.

But there was a modified version on a forum thread somewhere, which did the trick - so I thought I'd post it here for ease.

class WindowsFormsHostEx : WindowsFormsHost
{
    private PresentationSource _presentationSource;

    public WindowsFormsHostEx()
    {
        PresentationSource.AddSourceChangedHandler(this, SourceChangedEventHandler);
    }

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

        if (ParentScrollViewer == null)
            return;

        GeneralTransform tr = RootVisual.TransformToDescendant(ParentScrollViewer);
        var scrollRect = new Rect(new Size(ParentScrollViewer.ViewportWidth, ParentScrollViewer.ViewportHeight));

        var intersect = Rect.Intersect(scrollRect, tr.TransformBounds(rcBoundingBox));
        if (!intersect.IsEmpty)
        {
            tr = ParentScrollViewer.TransformToDescendant(this);
            intersect = tr.TransformBounds(intersect);
        }
        else
            intersect = new Rect();

        int x1 = (int)Math.Round(intersect.Left);
        int y1 = (int)Math.Round(intersect.Top);
        int x2 = (int)Math.Round(intersect.Right);
        int y2 = (int)Math.Round(intersect.Bottom);

        SetRegion(x1, y1, x2, y2);
    }

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

        if (disposing)
            PresentationSource.RemoveSourceChangedHandler(this, SourceChangedEventHandler);
    }

    private void SourceChangedEventHandler(Object sender, SourceChangedEventArgs e)
    {
        ParentScrollViewer = FindParentScrollViewer();
    }

    private ScrollViewer FindParentScrollViewer()
    {
        DependencyObject vParent = this;
        ScrollViewer parentScroll = null;
        while (vParent != null)
        {
            parentScroll = vParent as ScrollViewer;
            if (parentScroll != null)
                break;

            vParent = LogicalTreeHelper.GetParent(vParent);
        }
        return parentScroll;
    }

    private void SetRegion(int x1, int y1, int x2, int y2)
    {
        SetWindowRgn(Handle, CreateRectRgn(x1, y1, x2, y2), true);
    }

    private Visual RootVisual
    {
        get
        {
            if (_presentationSource == null)
                _presentationSource = PresentationSource.FromVisual(this);

            return _presentationSource.RootVisual;
        }
    }

    private ScrollViewer ParentScrollViewer { get; set; }

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

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