Minecraft 1.9.4
Forge 1.9.4-12.17.0.1932
Quark beta 21

For example, although a sticky piston will push a seated player forward, it will not pull them backward upon retracting.

pistons have never pulled entities in Minecraft (except for the piston translocation bug)

Yes, and players have never been able to sit on stairs. Now, players can and pistons should.

I do not understand how sitting on stairs has anything to do with how pistons work. The two are completely unrelated.

If you attach stairs to a sticky piston, sit on the stairs, and extend and retract the piston, you will be floating. There's no way in hell that's intended.

so you're saying it should kick you off the stairs?

You should retract with the stairs instead of floating, obviously. You are attached, after all.

he has a point. Slime Blocks, when pushing a block into the player, will retract them. Retracting sitting players does seem like correct behavior with that in mind.

Bit of a mess with this one because piston code is the worst thing ever. I think I'll just make it pop you off.