As said. Summon for example a pig and edit a sign while having the pig in sight. If the pig is moving when you start editing, it stops.
MC-155087
Mobs freeze when editing a sign
Resolved
Works As Intended
0
(Unassigned)
Unconfirmed
(Unassigned)
edit, freeze, mob, sign
Minecraft 1.14.3 Pre-Release 4
Retrieved
Just like the pause menu pauses the game in single player mode, so does the sign editing GUI.