When a Wither is in pursuit of a player or mob, and there are blocks in the way, it slowly float down to the floor despite not actually moving.

Steps to Reproduce:
1. Make a new Creative world.
2. Construct a Nether Portal and enter it.
3. Find a low ceiling similar to the ones in the pictures below.
4. Spawn a pig in the back part of the overhang.
5. Spawn a Wither where it will not blow up the overhang.
6. When the Wither starts flying against the blocks of the low ceiling (another bug reported in MC-8630), it will begin to sink towards the floor. If you attempt to hit the Wither when it is on the floor, it will not work.