That's not necessarily true. If your toolkit and compositor don't agree on whether you're using client-side or server-side decorations, then you're going to see either double decorations or no decorations.

Most of the core Wayland team believes that client-side decorations is the technically superior approach for efficiency and accuracy reasons, and we're disappointed KDE is still pushing for server-side decorations.

There is currently no protocol to negotiate between the toolkit and compositor whether to use client-side or server-side decorations. And I can't see one being added in the near future, either.