C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;
using System.Windows.Forms;
 
 
namespace Overlay {
  public class OverlayForm : Form {
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
      public int left;
      public int top;
      public int right;
      public int bottom;
    }
 
    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool GetWindowRect(HandleRef hwnd, out RECT lpRect);
 
    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool GetClientRect(HandleRef hwnd, out RECT lpRect);
 
    [DllImport("user32.dll")]
    public static extern IntPtr SetParent(HandleRef childWwnd, HandleRef newParentHwnd);
 
    private IntPtr parentHwnd;
 
    public OverlayForm(IntPtr parentHwnd) {
      this.parentHwnd = parentHwnd;
      this.Paint += new PaintEventHandler(OverlayForm_Paint);
 
      this.SuspendLayout();
      this.BackColor = System.Drawing.Color.Lime;
      this.FormBorderStyle = FormBorderStyle.None;
      this.TransparencyKey = Color.Lime;
      this.StartPosition = FormStartPosition.Manual;
      SetParent(new HandleRef(null, this.Handle), new HandleRef(null, parentHwnd));
      adjustSize();
      this.ResumeLayout(false);
      this.PerformLayout();
 
      Thread adjustThread = new Thread(adjustSizeContinuous);
      adjustThread.Start();
    }
 
    private void adjustSizeContinuous() {
      while(true) {
        this.SuspendLayout();
        adjustSize();
        this.ResumeLayout(false);
        this.PerformLayout();
 
        Thread.Sleep(500);
      }
    }
 
    private void adjustSize() {
      RECT windowRect = new RECT();
      RECT clientRect = new RECT();
      GetWindowRect(new HandleRef(null, parentHwnd), out windowRect);
      GetClientRect(new HandleRef(null, parentHwnd), out clientRect);
 
      this.Location = new Point(windowRect.left, windowRect.top);
      this.Size = new Size(clientRect.right, clientRect.bottom);
      Refresh();
    }
 
    private void OverlayForm_Paint(object sender, PaintEventArgs e) {
      Console.WriteLine("  cr = " + e.ClipRectangle);
 
      Graphics g = e.Graphics;
      g.Clear(Color.Lime);
      Pen pen = new Pen(Color.Red, 10);
      g.DrawRectangle(pen, 0, 0, ClientSize.Width, ClientSize.Height);
      g.Dispose();
    }
 
    public static void Main() {
      Application.EnableVisualStyles();
      Application.SetCompatibleTextRenderingDefault(false);
      IntPtr hwnd = new IntPtr(0x00D3024A);
      Application.Run(new OverlayForm(hwnd));
    }
  }
}