constraints

Use Expanded when a child must fill its share of a Row or Column, and Flexible when it may be smaller than its share. Measured layouts on Flutter 3.44 show where the leftover space goes, why loose children never donate it to siblings, and when only Flexible survives unbounded constraints.

A widget asked for height: double.infinity inside a parent with no height limit, like a Column or ListView. Use Expanded, a finite height, LimitedBox, or SliverFillRemaining instead.

Why a ListView in a Column throws 'Vertical viewport was given unbounded height', and the four fixes (Expanded, Flexible, shrinkWrap, SizedBox) with the performance trade-offs that decide which one you want.