Focus tree - A tree of focus nodes that typically sparsely mirrors the
widget tree, representing all the widgets that can receive focus.

Focus node - A single node in a focus tree. This node can receive the
focus, and is said to “have focus” when it is part of the focus chain. It
participates in handling key events only when it has focus.

Primary focus - The farthest focus node from the root of the focus tree
that has focus. This is the focus node where key events start propagating to
the primary focus node and its ancestors.

Focus chain - An ordered list of focus nodes that starts at the primary
focus node and follows the branches of the focus tree to the root of the
focus tree.

Focus scope - A special focus node whose job is to contain a group of
other focus nodes, and allow only those nodes to receive focus. It contains
information about which nodes were previously focused in its subtree.