Prior to this behavioral flag, the flexible header always assumed that it was presented in a full-screen capacity, meaning it would be placed directly behind the status bar or device bezel (such as the iPhone X's notch). This assumption does not support extensions and iPad popovers.
Enabling the inferTopSafeAreaInsetFromViewController flag tells the flexible header to use its view controller ancestry to extract a safe area inset from its context, instead of relying on assumptions about placement of the header.
This behavioral flag is disabled by default, but will eventually be enabled by default and the flag will eventually be removed.
flexibleHeaderViewController.inferTopSafeAreaInsetFromViewController = true
flexibleHeaderViewController.inferTopSafeAreaInsetFromViewController = YES;
Note: if this flag is enabled and you've also provided a topLayoutGuideViewController, take care that the topLayoutGuideViewController is not a direct ancestor of the flexible header or your app will enter an infinite loop. As a general rule, your topLayoutGuideViewController should be a sibling to the flexible header.