Added a hint to retain window content during state changes

In my testing window content is always retained, but this makes our intent explicit.

src/video/windows/SDL_windowswindow.c

@@ -337,6 +337,11 @@ static int SetupWindowData(_THIS, SDL_Window *window, HWND hwnd, HWND parent, SD
     data->initializing = SDL_TRUE;
     data->last_displayID = window->last_displayID;
     data->scaling_dpi = WIN_GetScalingDPIForHWND(videodata, hwnd);
+    if (SDL_GetHintBoolean("SDL_WINDOW_RETAIN_CONTENT", SDL_FALSE)) {
+        data->copybits_flag = 0;
+    } else {
+        data->copybits_flag = SWP_NOCOPYBITS;
+    }
 
 #ifdef HIGHDPI_DEBUG
     SDL_Log("SetupWindowData: initialized data->scaling_dpi to %d", data->scaling_dpi);
@@ -386,7 +391,7 @@ static int SetupWindowData(_THIS, SDL_Window *window, HWND hwnd, HWND parent, SD
                 /* Figure out what the window area will be */
                 WIN_AdjustWindowRect(window, &x, &y, &w, &h, SDL_FALSE);
                 data->expected_resize = SDL_TRUE;
-                SetWindowPos(hwnd, HWND_NOTOPMOST, x, y, w, h, SWP_NOCOPYBITS | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
+                SetWindowPos(hwnd, HWND_NOTOPMOST, x, y, w, h, data->copybits_flag | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
                 data->expected_resize = SDL_FALSE;
             } else {
                 window->w = w;
@@ -788,12 +793,12 @@ void WIN_SetWindowPosition(_THIS, SDL_Window *window)
      * the window to resize (e.g. AdjustWindowRectExForDpi frame sizes are different).
      */
     WIN_ConstrainPopup(window);
-    WIN_SetWindowPositionInternal(window, SWP_NOCOPYBITS | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
+    WIN_SetWindowPositionInternal(window, window->driverdata->copybits_flag | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
 }
 
 void WIN_SetWindowSize(_THIS, SDL_Window *window)
 {
-    WIN_SetWindowPositionInternal(window, SWP_NOCOPYBITS | SWP_NOMOVE | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
+    WIN_SetWindowPositionInternal(window, window->driverdata->copybits_flag | SWP_NOMOVE | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
 }
 
 int WIN_GetWindowBordersSize(_THIS, SDL_Window *window, int *top, int *left, int *bottom, int *right)
@@ -886,7 +891,7 @@ void WIN_ShowWindow(_THIS, SDL_Window *window)
 
     if (window->parent) {
         /* Update our position in case our parent moved while we were hidden */
-        WIN_SetWindowPositionInternal(window, SWP_NOCOPYBITS | SWP_NOACTIVATE);
+        WIN_SetWindowPositionInternal(window, window->driverdata->copybits_flag | SWP_NOACTIVATE);
     }
 
     hwnd = window->driverdata->hwnd;
@@ -987,7 +992,7 @@ void WIN_SetWindowBordered(_THIS, SDL_Window *window, SDL_bool bordered)
 
     data->in_border_change = SDL_TRUE;
     SetWindowLong(hwnd, GWL_STYLE, style);
-    WIN_SetWindowPositionInternal(window, SWP_NOCOPYBITS | SWP_FRAMECHANGED | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
+    WIN_SetWindowPositionInternal(window, data->copybits_flag | SWP_FRAMECHANGED | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOACTIVATE);
     data->in_border_change = SDL_FALSE;
 }
 
@@ -1101,7 +1106,7 @@ void WIN_SetWindowFullscreen(_THIS, SDL_Window *window, SDL_VideoDisplay *displa
     }
     SetWindowLong(hwnd, GWL_STYLE, style);
     data->expected_resize = SDL_TRUE;
-    SetWindowPos(hwnd, top, x, y, w, h, SWP_NOCOPYBITS | SWP_NOACTIVATE);
+    SetWindowPos(hwnd, top, x, y, w, h, data->copybits_flag | SWP_NOACTIVATE);
     data->expected_resize = SDL_FALSE;
 
 #ifdef HIGHDPI_DEBUG
@@ -1329,7 +1334,7 @@ void WIN_OnWindowEnter(_THIS, SDL_Window *window)
     }
 
     if (window->flags & SDL_WINDOW_ALWAYS_ON_TOP) {
-        WIN_SetWindowPositionInternal(window, SWP_NOCOPYBITS | SWP_NOSIZE | SWP_NOACTIVATE);
+        WIN_SetWindowPositionInternal(window, data->copybits_flag | SWP_NOSIZE | SWP_NOACTIVATE);
     }
 }
 

src/video/windows/SDL_windowswindow.h

@@ -75,6 +75,9 @@ struct SDL_WindowData
      * between dpi-scaled points and pixels. Only used if videodata->dpi_scaling_enabled.
      */
     int scaling_dpi;
+
+    /* Whether we retain the content of the window when changing state */
+    UINT copybits_flag;
 };
 
 extern int WIN_CreateWindow(_THIS, SDL_Window *window);