crossfade-container-sizes-to-the-visible-child

A crossfade's container is a Box aligned to the top-start corner and sized to the largest child currently composed, so swapping a thin child for a taller one pins the thin one to the top mid-transition and drops it when the tall one leaves. Covers boxing both branches into one fixed frame with an explicit alignment, replacing rather than stacking two progress renderers whose track lengths differ, and why fading a…