columnFlex property
Flex ratios for proportional column width distribution.
Example: [1, 2, 1] creates three columns where the middle column is twice
as wide as the others. Values are relative, so [1, 2, 1] and [2, 4, 2] produce identical layouts.
Length must equal headers.length. Assertion enforces this at construction time.
Implementation
final List<int> columnFlex;