A child set to height: 100% collapses to its content size instead of filling the parent. Here's the containing-block rule causing it, the two things everyone tries first, and four real fixes.
position: sticky not working? It's one of five causes: no top value, a clipping ancestor, a short parent, a stretched item, or html + body overflow. Fix each.
Position: sticky not working? Six causes ranked by how often they bite: a missing top, an overflow ancestor, flex stretch, a short parent, and one console snippet that finds it.
A flex child won't shrink and text-overflow: ellipsis silently does nothing? It's the automatic minimum size (min-width: auto) — here's the fix.