The minimum size of a ScrollPane is the size of the insets
plus minimum size of the viewport, plus the scrollpane's
viewportBorder insets, plus the minimum size
of the visible headers, plus the minimum size of the
scrollbars whose displayPolicy isn't NEVER.
Defines the requirements for a tree node object that can change --
by adding or removing child nodes, or by changing the contents
of a user object stored in the node.