Explain the rule constraints go down, sizes go up in Flutter.

Short answer

The parent sends constraints to the child, the child picks a size within them, and then the parent positions it.

Explanation

Solving overflow problems starts by reading the actual constraints, not by adding Expanded at random. A widget does not choose a size outside the limit its parent gave it, and some parents such as Column may send unbounded constraints on a particular axis.

Common mistakes

• Assuming the child imposes its size on the parent.
• Hiding overflow with a SingleChildScrollView without understanding the cause of the constraints.

Follow-up questions

• How do keys help Flutter preserve widget identity?

Sources

• Flutter docs — Understanding constraints