#flex

Trailing children of a tall absolutely-positioned flex column silently vanish (no overflow, no compression) because Yoga's auto-height under-measures the content. Fix: give the column an explicit height.

In s&box Yoga, an explicit flex-basis (notably flex-basis: 0 inherited from a base rule) takes precedence over width for a flex item's main-axis size: a fixed column collapses to zero.

A flex-grow slider track holding a normal-flow fill child sized by width:N% balloons wider as the value increases: take the fill out of flow with position:absolute, and make sure the track carries no overflow:hidden.