Arrange the children within availableArea.
The natural size this sizer would like, given its children.
Add a widget child and return its SizerItem for fluent configuration: box.add(w).proportion(1).pad(Padding.all(8)).alignV(VAlign.middle). A bare add(w) gives a non-stretching child (proportion 0) with no padding.
Add a nested sizer and return its SizerItem for fluent configuration.
Number of child items.
Lays children out left to right.