@ayyoub-coder gave a fantastic summary of the problem in #160177: if a
`Container` doesn't have a child or unbounded constraints, it will
expand to fill its parent, whereas a `ColoredBox` defaults to zero size.
I also noticed that in the main branch, the `PositionedDirectional`
widget has an unused `width` parameter, and instead builds a `SizedBox`
child for it.
This pull request cleans up the `Stepper` subtree a bit and allows the
connector to display properly.
fixes#160156
7aa4d2d3ec