Player height miscalculated when riding

When mounted, you can ride into a tunnel two blocks high. This causes the player head to penetrate the tiles at height of three tiles, so you only see deep black while in the tunnel.

Repro:

Mine a tunnel that's two blocks high. Mount a horse. Ride into the tunnel. You should see deep black.

Expected behaviour:

Player should collide with blocks at 3 block height when mounted.