The "Make Panel Toggle" operator does not check if active item is an input socket,
leading to a situation where it can be called on Boolean output sockets.
Doing this creates weird behavior where the panel and associated
operators believe it has a toggle, but is not reflected in the UI drawing.
Other weird behaviors include being able to rearrange output sockets
and apply the operator again to another output socket.
This patch adds a check in the operator's poll function to prevent it
from being called on output sockets.
Pull Request: https://projects.blender.org/blender/blender/pulls/137526
d3639828f8