This post is from the Activity iPhone Duo UIKit Q&A.
When the user transitions from an open to a partially folded state, the system detects this is the final state after some delay and un-blurs the left side.
The UI should now adapt to this state. For example, split view controller enlarges the sidebar to occupy the full width of the left side.
What’s the best trigger to use for a custom view controller that wants to achieve a similar result? Is it the folding region becoming active?