isHoisting method
Whether route should leave its chrome to the shell this frame.
True for every registered route while the shell is drawing, not only the topmost one: mid-transition the chrome is a blend of two routes' items, so the outgoing route has to keep its placeholder or its own buttons would slide out from underneath the pinned copy.
Implementation
bool isHoisting(ModalRoute<dynamic> route) {
if (!isActive || !_registry.containsKey(route)) return false;
final ordered = _orderedEntries;
if (ordered.isEmpty) return false;
return !_isPresentedOver(ordered.first.key);
}