x11: SDL_SetWindowPosition should try to wait for the window manager.

Wait up to 100 milliseconds, since the window manager might alter or
outright veto the window change...or not respond at all.

In a well-functioning system, though, this should help make sure
that SDL_SetWindowPosition's results match reality.

Fixes Bugzilla #4646.

src/video/x11/SDL_x11window.c

@@ -809,20 +809,36 @@ X11_SetWindowPosition(_THIS, SDL_Window * window)
     Window childReturn, root, parent;
     Window* children;
     XWindowAttributes attrs;
+    int orig_x, orig_y;
+    Uint32 timeout;
 
-    /*Attempt to move the window*/
-    X11_XMoveWindow(display, data->xwindow, window->x - data->border_left, window->y - data->border_top);
     X11_XSync(display, False);
-
-    /*If the window is not moved, then the coordinates on the window structure are out of sync, so we
-      update them here. */
     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,
-                              &window->x, &window->y,
-                              &childReturn);
+    X11_XTranslateCoordinates(display, parent, DefaultRootWindow(display),
+                              attrs.x, attrs.y, &orig_x, &orig_y, &childReturn);
+
+    /*Attempt to move the window*/
+    X11_XMoveWindow(display, data->xwindow, window->x - data->border_left, window->y - data->border_top);
+
+    /* Wait a brief time to see if the window manager decided to let this move happen.
+       If the window changes at all, even to an unexpected value, we break out. */
+    timeout = SDL_GetTicks() + 100;
+    do {
+        int x, y;
+        X11_XSync(display, False);
+        X11_XGetWindowAttributes(display, data->xwindow, &attrs);
+        X11_XTranslateCoordinates(display, parent, DefaultRootWindow(display),
+                                  attrs.x, attrs.y, &x, &y, &childReturn);
+
+        if ((x != orig_x) || (y != orig_y)) {
+            window->x = x;
+            window->y = y;
+            break;  /* window moved, time to go. */
+        }
+
+        SDL_Delay(10);
+    } while (!SDL_TICKS_PASSED(SDL_GetTicks(), timeout));
 }
 
 void