The flexible header allows you to forward touch events to the tracking scroll view. This provides the illusion that the flexible header is part of the tracking scroll view.
To start touch forwarding you must call forwardTouchEventsForView: with each view:
headerView.forwardTouchEvents(for: someContentView)
[headerView forwardTouchEventsForView:someContentView];
To stop touch forwarding you must call forwardTouchEventsForView: with each view:
headerView.stopForwardingTouchEvents(for: someContentView)
[headerView stopForwardingTouchEventsForView:someContentView];