Fix popup menu size when multiple windows UI is disabled

Fixed regression introduced in ed595eb6b1

Reported:
- https://community.aseprite.org/t/12497

src/ui/menu.cpp

@@ -365,6 +365,8 @@ void Menu::showPopup(const gfx::Point& pos,
   menubox->setMenu(this);
   menubox->startFilteringMouseDown();
 
+  window->remapWindow();
+
   fit_bounds(parentDisplay,
              window.get(),
              gfx::Rect(pos, window->size()),