Fix OpenTTD#9269, f6d5c01: Skip invalid windows when checking z-priority

src/window.cpp

@@ -1351,7 +1351,8 @@ static void BringWindowToFront(Window *w, bool dirty)
 {
 	auto priority = GetWindowZPriority(w->window_class);
 	WindowList::iterator dest = _z_windows.begin();
-	while (dest != _z_windows.end() && (*dest == nullptr || GetWindowZPriority((*dest)->window_class) <= priority)) ++dest;
+	/* Dropdowns and Pickers use WC_INVALID to hide themselves until deletion. */
+	while (dest != _z_windows.end() && (*dest == nullptr || (*dest)->window_class == WC_INVALID || GetWindowZPriority((*dest)->window_class) <= priority)) ++dest;
 
 	if (dest != w->z_position) {
 		_z_windows.splice(dest, _z_windows, w->z_position);