On iOS 27, scrolling up through a thread now minimizes the navigation bar
the way Messages does, and scrolling back toward the latest message
restores it. The thread route sets the header's minimizationBehavior to
onScrollUp.

UIKit only minimizes against a scroll view registered with the screen's
controller. The feed now sits in a ScrollViewMarker, and Screens 5.0.0-t3.37
makes Split.Screen (which hosts every workspace route) accept that
registration.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>