x11: Wait a bit to see if window pos changes when changing fullscreen.

Helps prevent window from moving to 0,0 when leaving fullscreen.

Fixes #4749.

src/video/x11/SDL_x11window.c

@@ -1312,6 +1312,20 @@ X11_SetWindowFullscreenViaWM(_THIS, SDL_Window * window, SDL_VideoDisplay * _dis
 
     if (X11_IsWindowMapped(_this, window)) {
         XEvent e;
+        /* !!! FIXME: most of this waiting code is copy/pasted from elsewhere. */
+        int (*prev_handler) (Display *, XErrorEvent *) = NULL;
+        unsigned int childCount;
+        Window childReturn, root, parent;
+        Window* children;
+        XWindowAttributes attrs;
+        int orig_x, orig_y;
+        Uint64 timeout;
+
+        X11_XSync(display, False);
+        X11_XQueryTree(display, data->xwindow, &root, &parent, &children, &childCount);
+        X11_XGetWindowAttributes(display, data->xwindow, &attrs);
+        X11_XTranslateCoordinates(display, parent, DefaultRootWindow(display),
+                                  attrs.x, attrs.y, &orig_x, &orig_y, &childReturn);
 
         if (!(window->flags & SDL_WINDOW_RESIZABLE)) {
             /* Compiz refuses fullscreen toggle if we're not resizable, so update the hints so we
@@ -1361,6 +1375,39 @@ X11_SetWindowFullscreenViaWM(_THIS, SDL_Window * window, SDL_VideoDisplay * _dis
             X11_XSendEvent(display, RootWindow(display, displaydata->screen), 0,
                    SubstructureNotifyMask | SubstructureRedirectMask, &e);
         }
+
+        /* Wait a brief time to see if the window manager decided to let this happen.
+           If the window changes at all, even to an unexpected value, we break out. */
+        X11_XSync(display, False);
+        prev_handler = X11_XSetErrorHandler(X11_CatchAnyError);
+
+        timeout = SDL_GetTicks64() + 100;
+        while (SDL_TRUE) {
+            int x, y;
+
+            caught_x11_error = SDL_FALSE;
+            X11_XSync(display, False);
+            X11_XGetWindowAttributes(display, data->xwindow, &attrs);
+            X11_XTranslateCoordinates(display, parent, DefaultRootWindow(display),
+                                      attrs.x, attrs.y, &x, &y, &childReturn);
+
+            if (!caught_x11_error) {
+                if ((x != orig_x) || (y != orig_y)) {
+                    window->x = x;
+                    window->y = y;
+                    break;  /* window moved, time to go. */
+                }
+            }
+
+            if (SDL_GetTicks64() >= timeout) {
+                break;
+            }
+
+            SDL_Delay(10);
+        }
+
+        X11_XSetErrorHandler(prev_handler);
+        caught_x11_error = SDL_FALSE;
     } else {
         Uint32 flags;