Anything moving while opening menu screen cause this
When the game is paused while the player is moving, the background goes jittery and "vibrates". Note this does not happen while the player is stationary.
How to replicate:
Simply open a single player world in Minecraft 1.12-pre6 and move around. This could be through walking, sprinting, flying, looking around/rotating or jumping. If you pause the game during movement, you will experience it.
- N.B. If you aren't experiencing the bug it may either only happen for me, the bug was fixed or certain conditions weren't filled to replicate the bug.
It might also not affect only 1.12-pre6, but other versions as well.