unbroken height (children + padding both sides) — when the box actually splits, each extra segment adds its own padding so the real total can exceed this; an approximation is fine here (same precedent as TableBlock.measureHeight) and it keeps the value safe for confined parents (row/box) that reserve exactly this much and never split
a breakable box only needs room for its first child (+ padding) to start on the current page — without this, the engine's break-before check (which uses full measureHeight) would push a long box whole to a fresh page and the mid-page split would never happen; a keepTogether box genuinely needs its full height, so it reports exactly that
draw onto the doc; must not compute page-breaks itself — use ctx.ensureSpace
A painted container around a vertical stack of children. Default mode splits across pages at child boundaries, "clone" style: measure children up front (measure === render for every allowed child, so segment heights are known before anything is drawn — which is what lets the background be painted before its content despite jsPDF having no z-order), greedily take children that fit the space left on the current page, paint that segment's full box (background + border + its own padding), render the children into a confined sub-context, then forcePageBreak and continue with a fresh box at the top of the next page.
keepTogetheropts out: the whole box moves to the next page instead, and content taller than one full page throws. In both modes a single child taller than one full page throws (no segmentation can place it) — fail-fast instead of silently drawing past the content area.