x11: Wait a bit in SDL_SetWindowSize() to see if window manager vetoed change.

Same idea as the fix for Bugzilla #4646.

Fixes Bugzilla #4727.

src/video/x11/SDL_x11window.c

@@ -910,6 +910,14 @@ X11_SetWindowSize(_THIS, SDL_Window * window)
 {
     SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
     Display *display = data->videodata->display;
+    XWindowAttributes attrs;
+    int orig_w, orig_h;
+    Uint32 timeout;
+
+    X11_XSync(display, False);
+    X11_XGetWindowAttributes(display, data->xwindow, &attrs);
+    orig_w = attrs.width;
+    orig_h = attrs.height;
 
     if (SDL_IsShapedWindow(window)) {
         X11_ResizeWindowShape(window);
@@ -953,7 +961,27 @@ X11_SetWindowSize(_THIS, SDL_Window * window)
         X11_XResizeWindow(display, data->xwindow, window->w, window->h);
     }
 
-    X11_XFlush(display);
+    /* Wait a brief time to see if the window manager decided to let this resize happen.
+       If the window changes at all, even to an unexpected value, we break out. */
+    timeout = SDL_GetTicks() + 100;
+    while (SDL_TRUE) {
+        X11_XSync(display, False);
+        X11_XGetWindowAttributes(display, data->xwindow, &attrs);
+
+        if ((attrs.width != orig_w) || (attrs.height != orig_h)) {
+            window->w = attrs.width;
+            window->h = attrs.height;
+            break;  /* window changed, time to go. */
+        } else if ((attrs.width == window->w) && (attrs.height == window->h)) {
+            break;  /* we're at the place we wanted to be anyhow, drop out. */
+        }
+
+        if (SDL_TICKS_PASSED(SDL_GetTicks(), timeout)) {
+            break;
+        }
+
+        SDL_Delay(10);
+    }
 }
 
 int