The are that's going to covered by the next tile when stacked is well known in advance. Obviously solar cells are not installed in this area. As for the shading

The are that's going to covered by the next tile when stacked is well known in advance. Obviously solar cells are not installed in this area. As for the shading from leaves or something, typically diodes are installed in solar panels to prevent current flow into the shaded parts. This helps mitigate the power loss from shades.