Flex

Flex boxes themselves (Row and Column)
behave differently based on whether they are in
bounded constraints or unbounded constraints in
their given direction.

In bounded constraints,
they try to be as big as possible in that direction.

In unbounded constraints,
they try to fit their children in that direction.
In this case, you cannot set flex on the children to
anything other than 0.
In the widget library, this means that you cannot use
Expanded when the flex box is inside
another flex box or inside a scrollable. If you do,
you’ll get an exception message pointing you at this document.

In the cross direction, for example, in the width for
Column (vertical flex) or in the height for
Row (horizontal flex), they must never be unbounded,
otherwise they would not be able to reasonably align their children.