The bug

When you press the place/interact key with a slab item on a bottom slab, it tries to place it, even when that action collides with an entity. Meaning: It plays the animation and off-hand items can't be used.

How to reproduce

1. Slab on floor
2. Slab in main hand
3. Bow/food/whatever in off-hand
4. Stand on slab
5. Look down
6. Press action key (standard: right click)
   → Bow/food doesn't get triggered, but slab, although it can't be placed.

Code analysis

Code analysis by marcono1234 can be found in this comment.