firstChild property

The first child of the viewport according to the traversal order of the mainAxis.

The mainAxis correlates with the ChildVicinity of each child to paint the children in a row or column major order.

By default, the mainAxis is Axis.vertical, which would result in a row major paint order, visiting children in the horizontal indices before advancing to the next vertical index.

This value is null during layoutChildSequence as children are reified into the correct order after layout is completed. This can be used when overriding paint in order to paint the children in the correct order.