Display areas don't occur directly in lines.
Display area has an inherent orientation. Think of this as an arrow
saying "this should align with filling direction".
Display areas are used to fill area containers.
Result of filling area container is area.
Area container has filling direction. Size of area container can grow
in this direction according to size of areas with which it is filled.
Display areas are stacked in filling direction.
Size of area container in other direction fixed.
Filling direction of area container = placement direction of display
A display area is always created so that it's size in direction
perpendicular to its placement direction is equal to size of area
container in direction perpendicular to filling direction. This is
the display size of the area.
Area container can specify rotation to be applied to display areas
before they are filled.