The square is expanded here rather than in StatResid because a stat runs before the position: four corners jittered one row at a time tear apart. It is also the only place the panel's final ranges are known, and the square is a square on the page rather than in data units.
Format
A ggplot2::Geom object.
See also
gf_square_resid(), which pairs this geom and StatResid for you.