Embedding window (of another process) inside another on win 10

<Window x:Class="EmbedWindow.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Embedded window example" Height="350" Width="525" Loaded="Window_Loaded" SizeChanged="Window_SizeChanged">
    <DockPanel>
        <Grid Name="myContent" DockPanel.Dock="Right">
            <TextBlock Margin="10">Some extra content here...</TextBlock>
        </Grid>
        <Border Name="childPlaceholder" BorderBrush="DarkGray" BorderThickness="2" >
            <TextBlock Margin="10">Here should be the embedded window...</TextBlock>
        </Border>
    </DockPanel>
</Window>

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Interop;

namespace EmbedWindow
{
	public partial class MainWindow : Window
	{
		IntPtr otherWindow = IntPtr.Zero, thisWindow = IntPtr.Zero;

		public MainWindow()
		{
			InitializeComponent();
		}

		private async void Window_Loaded(object sender, RoutedEventArgs e)
		{
			//This window's handle
			thisWindow = new WindowInteropHelper(this).Handle;

			//Open another program for embedding
			Process proc = new Process();
			proc.StartInfo = new ProcessStartInfo("Notepad.exe");
			proc.Start();

			//Wait until otherWindow has been opened
			await Task.Delay(500);

			otherWindow = proc.MainWindowHandle;

			//This bit removes the border of otherWindow and sets thisWindow as parent
			//I actually don't know what flags should be set, but simply setting the WS_VISIBLE flag seems to make window work, however borderless.
			WinHelper.SetWindowLong(otherWindow, WinHelper.GWL_STYLE, WinHelper.winStyle.WS_VISIBLE);
			WinHelper.SetParent(otherWindow, thisWindow);
			ArrangeWindows();
		}

		private void Window_SizeChanged(object sender, SizeChangedEventArgs e)
		{
			ArrangeWindows();
		}

		private void ArrangeWindows()
		{
			//Moves the otherWindow on top of childPlaceHolder
			Point topLeft = childPlaceholder.TransformToAncestor(this).Transform(new Point(0, 0));
			Point bottomRight = childPlaceholder.TransformToAncestor(this).Transform(new Point(childPlaceholder.ActualWidth, childPlaceholder.ActualHeight));
			WinHelper.MoveWindow(otherWindow, (int)topLeft.X, (int)topLeft.Y, (int)bottomRight.X - (int)topLeft.X, (int)bottomRight.Y - (int)topLeft.Y, true);
		}

		//Some imported functions from user32.dll
		static class WinHelper
		{
			//Sets a window to be a child window of another window
			[DllImport("user32.dll")]
			public static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);

			[DllImport("user32.dll")]
			public static extern bool MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

			//Sets window attributes
			[DllImport("user32.dll")]
			public static extern int SetWindowLong(IntPtr hWnd, int nIndex, winStyle dwNewLong);

			public static int GWL_STYLE = -16;

			[Flags]
			public enum winStyle : int
			{
				WS_VISIBLE = 0x10000000,
				WS_CHILD = 0x40000000, //child window
				WS_BORDER = 0x00800000, //window with border
				WS_DLGFRAME = 0x00400000, //window with double border but no title
				WS_CAPTION = WS_BORDER | WS_DLGFRAME //window with a title bar
			}
		}
	}
}