Manages the focus tree.
The FocusManager is held by the WidgetsBinding as WidgetsBinding.focusManager. The FocusManager is rarely accessed directly. Instead, to find the FocusScopeNode for a given BuildContext, use FocusScope.of.
The FocusManager knows nothing about FocusNodes other than the one that is currently focused. If a FocusScopeNode is removed, then the FocusManager will attempt to focus the next FocusScopeNode in the focus tree that it maintains, but if the current focus in that FocusScopeNode is null, it will stop there, and no FocusNode will have focus.