For Design-time interactions with Custom Components, document how the Designer interacts with the component.
For example, what’s the component’s parent? Is that reference even valid?
Code always runs in some kind of environment. If the environment differs from the one it’s been programmed for, it’s going to misbehave, and distract the developer.
It’s one thing for a component to be able to tell which environment it’s running in. It’s something else for the developer to know how those environments differ, so that they can update the old components (or build new ones) to behave appropriately for each environment.