Sign in hotbar causes a Box Cursor (visual bug)

When there is a sign in your hot bar, the aiming cursor turns into a box (selected as your active or as a non-active).

If the sign is in your inventory, there is no visual bug.

Whether you are in creative/survival does not seem to change the effect. It seems to be visual only, your cursor is completely functional otherwise.

I am using the faithful texture pack, which was my immediate thoughts. However, why does it show only with a sign in hotbar?

(See attached images)