For testing I placed a WitherBoss spawner in my world in a in a bright room.
What I expected to happen was...:
Nothing.
What actually happened was...:
The Wither health bar appeared on the screen on the sky became darker, even if I'm not looking at the spawner.
This also overwrites the health bar of any existing Withers in the world.
Steps to Reproduce:
1. Place a Wither Spawner in your world in a bright room (use for example TooManyItems, MCedit or NBTedit)
2. Now the Healthbar will appear.
3. Walk or fly away from it and look in the opposite direction of the spawner.
4. Spawn a real Wither and deal some damage to it.
5. The health bar doesn't chance.
Comments 7
Confirmed in 1.8.3. Also occurs with Enderdragons. Relates to MC-78519.
Confirmed in 1.8.8. but fixed in 15w46a (not sure if earlier since i didn't check all of the snapshots)
P.S. In my humble opinion this ''Bug'' is/was an rather cool one which I think could have stayed, as it is very useful for servers. For example if you want to display a boss bar without having to worry about an Wither/EnderDragon the Spawner was a good alternative. (sorry if this part was a bit ''emotional'' but as a server dev i often use(d) this technic/bug)
Confirmed. If the wither in the spawner goes out of render distance or the spawner is not loaded, the effect disappears.