Example 26

If all of Row’s children are wrapped in Expanded widgets,
each Expanded has a size proportional to its flex parameter,
and only then each Expanded widget forces its child to have
the Expanded’s width.

In other words, Expanded ignores the preferred width of
its children.

Example 27

Example 28

The screen forces the Scaffold to be exactly the same size
as the screen, so the Scaffold fills the screen.
The Scaffold tells the Container that it can be any size it wants,
but not bigger than the screen.

Note:
  When a widget tells its child that it can be smaller than a
  certain size, we say the widget supplies loose constraints
  to its child. More on that later.

Example 29