this may be intended or planned to fix in 1.10, but either way it looks bad

I thought that if you cover the top halves of stairs with another slab, the bottom half of the partially covered stairs would appear lit

instead, it appeared unlit

to reproduce this,

1. make a thingy of wooden planks 3 blocks tall, 3 blocks wide, and 2 blocks long

2. on the 3 blocks wide side, in the middle block, put stairs in any configuration (the important thing here is that the stairs are completely covered on all sides except for the side facing you)

3. in front of those stairs, on the exposed side, put slabs or stairs in any configuration

you will now see that the original stairs are darkened

I understand that this is because the stairs are full blocks and because of this I don't know if this is exactly a bug, but as I said before, it doesn't look nice