Mir is not clipping the streams. I don't think the streams *should* be clipped to the main surface - there are sensible use-cases for allowing it. For example, GTK uses subsurfaces on Wayland to draw its decorations, and they're outside the main surface pretty much by definition.
I'd probably be able to be convinced that clipping the streams to the main surface would be a good idea, though.
Mir is not clipping the streams. I don't think the streams *should* be clipped to the main surface - there are sensible use-cases for allowing it. For example, GTK uses subsurfaces on Wayland to draw its decorations, and they're outside the main surface pretty much by definition.
I'd probably be able to be convinced that clipping the streams to the main surface would be a good idea, though.