If non-null, whether the node currently holds input focus.
At most one node in the tree should hold input focus at any point in time, and it should not be set to true if focusable is false.
Input focus indicates that the node will receive keyboard events. It is not to be confused with accessibility focus. Accessibility focus is the green/black rectangular highlight that TalkBack/VoiceOver draws around the element it is reading, and is separate from input focus.
final bool focused