A border is a split-supporting component that contains five border edges
(top,left, bottom, right, split). The split-border is not used for rendering,
it is meant to replace the other borders on inner split-edges.
The border object itself is immutable. During a split operation, new borders
have to be created.
The border-radius defines the corner-rounding that might take place. This
defines the applicable background area of the content box. (Round rects never
cause the background to overlap the border, the corner space that lies
outside the rounded corners will not receive the background.)
The radius *must* be normalized; the sum of the radius sizes for a single
edge must not exceed the edge's total size. (Ex: height >=
(topLeftRadius.getHeight() + bottomLeftRadius.getHeight()). If the height is
smaller as the radius, reduce the radius until both sizes fit.