EnterAlwaysCollapsed

Scrolling up collapses the header and then slides the toolbar off screen too, giving the body the whole viewport. Scrolling down brings the toolbar back immediately; the header expands only once the body is back at its top.

The body is measured to the viewport with the toolbar gone, so its last stretch is reachable once the toolbar has exited. Pair with snapOnRelease if a half-exited toolbar should never rest on screen.