Focus traversal - The process of moving from one focusable node to
another in a predictable order. This is typically seen in applications when
the user presses the Tab key to move to the next focusable control or
field.

FocusNode and FocusScopeNode

The FocusNode and FocusScopeNode objects implement the
mechanics of the focus system. They are long-lived objects (longer than widgets,
similar to render objects) that hold the focus state and attributes so that they
are persistent between builds of the widget tree. Together, they form
the focus tree data structure.