The direction in which the child is permitted to be larger than the viewport.
The child is given layout constraints that are fully unconstrained along the main axis (e.g., the child can be as tall as it wants if the main axis is vertical).
Source
Axis get mainAxis => _mainAxis;
Source
set mainAxis(Axis value) {
assert(value != null);
if (value == _mainAxis)
return;
assert(_offsetIsSane(_paintOffset, value));
_mainAxis = value;
markNeedsLayout();
}