The player appears slightly higher than the required position, when he opens the shulker box, located under the minecart, in which the player sits.

Edit: As of 18w30b shulker box with boat above it can no longer be opened, but you still can open it if minecart is on top of the shulker box.

Edit 2: As of 1.14 snapshots (or earlier) shulker box with the boat above can now be opened again

What I expected to happen was...:
The shulker box must not be opened. / The player, together with the minecart/boat, must rise correctly when the shulker box is opened.

What actually happened was...:
The player appears slightly higher than the required position, when he opens the shulker box.

Steps to reproduce:

1. Place the rail, then place the minecart on it, or use /summon minecart.
2. Place the shulker box nearby.
3. Roll the minecart onto the shulker box.
4. Sit in the minecart and open the shulker box.

Steps to reproduce _(var2)

1. Place the shulker box.
2. Place a boat on the center of shulker box.
3. Sit in the boat and open the shulker box.