Slab placement ignores non-full hitbox block, allowing unintended stacking and trap exploit

When placing a slab on top of a block that has a non-full hitbox (such as blocks with partial collision or clickable space on top), the game incorrectly allows the slab to be placed on a slab above it instead of interacting with the intended block surface.

This results in unintended slab stacking behavior, which can be exploited in trap designs to create unfair or misleading setups.

Steps to reproduce:

1. Place a full block (e.g., stone).

2. Place a block with a non-full hitbox on top of it (e.g., a block with partial collision or clickable top space).

3. Place a slab above that non-full hitbox block.

4. Hold a slab and right-click on the top surface of the non-full hitbox block.

Expected Result:

The slab not be placed or should be placed relative to the surface of the non-full hitbox block that the player is targeting.

Actual Result:

The slab is placed onto the slab above instead, effectively stacking or merging with it, even though the player is aiming at the lower block.

Impact:

• Enables misleading or unintended trap mechanics.

• Breaks consistency in block placement behavior.

• Can be used to gain an unfair advantage in multiplayer environments.
  

Additional Notes:

This behavior appears to prioritize slab merging/placement over accurate hitbox targeting when interacting with blocks that do not occupy a full cube.