On the Hayashi track the surface area decreases enormously. The protostar contracts at almost constant surface temperature. The contraction results in gravitational potential energy heating the protostellar interior *and* supplying the protostellar luminosity.

I am not sure what you mean by an optically thick "shell". Class I protostars are still accreting from an optically thick envelope and are not generally shown on an HR diagram. Protostars on the Hayashi track are either class II or class III protostars (or pre main sequence stars). These have visible photospheres and any  circumstellar matter is confined to a disc.