To reproduce, start a widget example i.e. wiggly on weston or pure-qml and drag the window borders. The Application will freeze after a few frames.
This regression happened in https://codereview.qt-project.org/#/c/208855/
Not all compositors send a number of release events that matches the number of attaches the client does, so the client hangs waiting for the compositor to release the buffer.
Gnome-shell and kwin sends multiple releases, but not Weston and Qt compositors do not. As far as I can tell, what should happen is undefined in the protocol (and racy in the Weston/Qt Wayland case), so we should try to figure out if we can avoid attaching multiple times.