There is a concept of major and minor UI differences:
– layout tweaks would be minor.
– event driven layout rules (closing windows, looking at windows, window order) would be minor.
You may want users to be able to make minor changes to UI, but not major, so that they become more efficient, and so that they don’t bug you to make small changes. That way, you can focus on This Stack.
Allowing the User to make only minor tweaks is important because that way users can use each other’s setups and still understand them. The way to measure whether something is major or minor is whether the change is small enough that users can still understand each other’s setups. In other words one user can’t change the UI so much that another user will be disoriented.
Major changes would be Orientation changes which would make different user’s setups hard for each other to understand and use. Minor changes would be called sub-orientation changes.