CB= Center of Buoyancy. It's the force counteracting gravity.

In a stable configuration, the horizontal position of the CB = horizontal position of the CG, otherwise the boat will shift until they do.

As the person leans out (to the right), the combined CG shifts to the right, as does the CB as the mast comes out of the water. The further the CG moves away from the CB, the faster the boat will move in an attempt to restore equilibrium.

The bottom line is that only the horizontal positions of the Center of Buoyancy and Center of Gravity matter. How they get to where they are makes no difference at all.