They cannot be closed with server decorations either; if the client doesn't process the even queue and ignores WM_DELETE_WINDOW, it doesn't matter who paints the decoration.

> for the compositor to find out if an application is unresponsive or not literally means solving the halting problem.

Not in Wayland; if the client doesn't process events, the compositor knows that. It is just a matter of deciding on timeout value and then letting the user know (mutter does that, for example).

---

However, client side decorations ho have much more severe implications beyond estetic. For starters, with server decorations, you cannot guarantee frame-perfect synchronization of decoration and client area painting; with client decorations, you can, the same process does it.