Verification builds:
1.2.0.15 Retail

Summary:
Since 1.2.0.15 (possibly earlier) the hit box for stairs has changed, and now behaves like a full block. If you stand at the side of a stair block, you cannot interact with anything above the lower step, as it acts as if it is full block height. This is not how stairs used to be in 1.1, and is not how they work in Java. (See pictures to explain)

Steps to Reproduce:

1. Place a stair block
2. Stand next to it
3. Place the crosshair just above the first step
4. Try to break the block next to the stair

Observed Results:
The stair block gets broken, as the hitbox remains in focus as a full sized block.

Expected Results:
You should be able to lose focus of the stair, if the crosshair is not actually pointed at the rendered part of the stair.

Screenshots/Videos attached: Yes

Notes:
Test world: Stair Hitbox.mcworld