I believe you. Can you share an example? To be clear, I'm pretty sure this can be done with Gtk and Qt, but maybe you are talking about older apps written directly in Xlib or Xt?
It's not the toolkit (as they work on X11 and other OSes with the existing toolkits), it's Wayland that's the issue (there are a series of Wayland protocols to implement this, but they are being blocked).
