The constraints are sometimes “tight”,
meaning that they leave no room for the render box to decide on
a size (for example, if the minimum and maximum width are the same,
it is said to have a tight width). An example of this is the
App widget, which is contained by the RenderView
class: the box used by the child returned by the
application’s build function is given a constraint
that forces it to exactly fill the application’s content area
(typically, the entire screen).
Many of the boxes in Flutter, especially those that just take a
single child, pass their constraint on to their children.
This means that if you nest a bunch of boxes inside each other
at the root of your application’s render tree,
they’ll all exactly fit in each other, forced by these tight constraints.

Some boxes loosen the constraints,
meaning the maximum is maintained but the
minimum is removed. For example, Center.

Unbounded constraints