Various actionbar messages overlap

The bug

Multiple actionbar-area messages can coexist. When a new message appears, it overlaps with previous messages instead of replacing the old one.

How to reproduce

1. Run the command /title @a actionbar command message repeatedly
2. Play a music disc in a jukebox
3. Start riding a pig
4. Observe the actionbar

Expected behavior

All messages are aligned to the same height, and any new message clears any old one.