no idea why LWA_COLORKEY is much more performance expensive than DwmExtendFrameIntoClientArea.

So going to use DwmExtendFrameIntoClientArea.

ClickableTransparentOverlay/Overlay.cs

@@ -40,10 +40,10 @@
             _future_size = Vector2.Zero;
             _future_pos = Vector2.Zero;
 
-            _window = new Sdl2Window("Overlay", x, x, width, height, SDL_WindowFlags.Borderless | SDL_WindowFlags.AlwaysOnTop | SDL_WindowFlags.Resizable | SDL_WindowFlags.SkipTaskbar, true);
+            _window = new Sdl2Window("Overlay", x, x, width, height, SDL_WindowFlags.Borderless | SDL_WindowFlags.AlwaysOnTop | SDL_WindowFlags.SkipTaskbar, true);
             // TODO: Create a new branch for Non-Veldrid dependent version. Ideally, we can directly use SDL2Window.
             _gd = VeldridStartup.CreateGraphicsDevice(_window, new GraphicsDeviceOptions(true, null, true), GraphicsBackend.Direct3D11);
-            WinApi.EnableTransparent(_window.Handle);
+            WinApi.EnableTransparent(_window.Handle, new System.Drawing.Rectangle(_window.X , _window.Y, _window.Width, _window.Height));
             _window.Resized += () =>
             {
                 _gd.MainSwapchain.Resize((uint)_window.Width, (uint)_window.Height);
@@ -89,8 +89,9 @@
             _future_pos.Y = y;
             _future_size.X = width;
             _future_size.Y = height;
-            // TODO: move it to _window.Moved
+            // TODO: move following two lines to _window.Moved
             _hook_controller.UpdateWindowPosition(x, y);
+            WinApi.EnableTransparent(_window.Handle, new System.Drawing.Rectangle(x, y, width, height));
             _require_resize = true;
         }
 

ClickableTransparentOverlay/WinApi.cs

@@ -14,13 +14,48 @@
         private const int SW_HIDE = 0x00;
         private const int SW_SHOW = 0x05;
 
-        public static void EnableTransparent(IntPtr handle)
+        public static void EnableTransparent(IntPtr handle, System.Drawing.Rectangle size)
         {
             int windowLong = GetWindowLong(handle, GWL_EXSTYLE) | WS_EX_LAYERED | WS_EX_TRANSPARENT;
             SetWindowLong(handle, GWL_EXSTYLE, new IntPtr(windowLong));
-            SetLayeredWindowAttributes(handle, 0, 255, LWA_ALPHA | LWA_COLORKEY);
+            //SetLayeredWindowAttributes(handle, 0, 255, LWA_ALPHA /*| LWA_COLORKEY*/ );
+            Margins margins = Margins.FromRectangle(size);
+            DwmExtendFrameIntoClientArea(handle, ref margins);
         }
 
+        #region Structures
+
+        [StructLayout(LayoutKind.Sequential)]
+        private struct Margins
+        {
+            private int left, right, top, bottom;
+
+            public static Margins FromRectangle(System.Drawing.Rectangle rectangle)
+            {
+                var margins = new Margins
+                {
+                    left = rectangle.Left,
+                    right = rectangle.Right,
+                    top = rectangle.Top,
+                    bottom = rectangle.Bottom
+                };
+                return margins;
+            }
+        }
+
+        [StructLayout(LayoutKind.Sequential)]
+        private struct Rect
+        {
+            private readonly int left, top, right, bottom;
+
+            public System.Drawing.Rectangle ToRectangle(System.Drawing.Point point)
+            {
+                return new System.Drawing.Rectangle(point.X, point.Y, right - left, bottom - top);
+            }
+        }
+
+        #endregion Structures
+
         public static void HideConsoleWindow()
         {
             var handle = GetConsoleWindow();
@@ -33,6 +68,9 @@
             ShowWindow(handle, SW_SHOW);
         }
 
+        [DllImport("dwmapi.dll")]
+        private static extern IntPtr DwmExtendFrameIntoClientArea(IntPtr hWnd, ref Margins pMarInset);
+
         [DllImport("user32.dll", SetLastError = true)]
         private static extern int GetWindowLong(IntPtr hWnd, int nIndex);