If a client has been resized, it doesn't necessarily mean that the
decoration theme will schedule full repaint of the window frame. In
OpenGL and Xrender scene, we have a little hack that forces a full
repaint of window borders. However, we don't have one in QPainter
scene which causes all sorts of weird looking artifacts when resizing
a server-side decorated client.
We could add yet another hack in the QPainter scene, but a better
approach to tackle this problem would be to make DecoratedClient
schedule full repaint of the decoration. It makes code in scene
plugins more straightforward and prevents us from repeating the same
mistake again.