X does allow this, but it needs cooperation from clients, which in practice means that Gtk and Qt need to implement it, and they decided to only implement this

X does allow this, but it needs cooperation from clients, which in practice means that Gtk and Qt need to implement it, and they decided to only implement this for Wayland (which also needs clients to implement this!) because for them, X is dead and not worth supporting. It's not a problem of X per se. It's a deliberate decision to not implement a feature for X.