There are two basic processes which happen in the VCL node tree –
redrawing of regions (down-propagating, running from the root to leaves)
and the propagation of a change (up-propagating, running from a leaf to the
root). The first process is initiated in the canvas (by a GTK expose event) and
managed by the current painter. There is a context structure which is
internally used by painters and which can take complete control of the
drawing process. Canvas calls the painter's methods and the painter walks
through the tree using tree-examining methods of containers, and draws
the appropriate nodes. The painter ignores uninteresting branches of the node
tree. Containers are responsible for storing information about their
children (they usually store the aggregate bounding box of all children)
needed to cooperate with the painter so as to avoid walking through uninteresting
branches. These data are updated during the second process –
propagation of a change.