What are the requirements for keeping someone as your vassal when they inherit/conquer lands outside your realm ?
The wiki says they most already hold an equal title in your realm, but I've had many counts inherit foreign duchies. But when a baron inherited a foreign duchy, he left my realm and took his barony with him. I then assumed the rule was they can only inherit a title one rank up from the one they currently have, but a duke inheriting a kingdom leaves your empire.
So from what I can tell, the rules are somewhat arbitrary. What are the requirements for keeping a vassal when they inherit a holding/county/duchy/kingdom?