ウィドウの規則で短くなった段を埋める

const balancing = {
  // A ragged column has no lever of its own when the widow and orphan rules hold its last
  // line back: no heading or list end to open, no justified paragraph to set looser. This
  // one lets a paragraph of the short column run a line long, within its own word spacing
  // (or a justified one no looser than maxWordSpacing), or, when none can, set one a line
  // short so the split after it takes the line. A change is kept only when the pages end
  // with fewer short columns; otherwise the column stays as it was.
  fillShortColumns: true, // default false
  // The paragraphs go first: a crosshead keeps its one line of air unless they cannot help.
  maxLinesBeforeLoose: 0,
};
const bodyText = {
  fontFamily: 'Public Sans', fontSize: pt(9.4), lineHeight: pt(LEAD), color: col('ink'),
  boldColor: col('ink'), italicColor: col('ink'), referenceColor: col('ink'),
  textAlign: 'left', firstLineIndent: pt(0), paragraphSpacing: true, // a grid line between
  hyphenation: { enabled: true }, optimalLineBreaking: true, optimalRagged: true,
  avoidWidows: true, widowMinLines: 2, avoidOrphans: true, orphanMinLines: 2, // the cause
  avoidRunts: true,
};

const withFill = (fill) => ({ ...config, headings: { ...config.headings,
  balancing: { ...balancing, fillShortColumns: fill } } }); // a copy: config keeps true
const build = (fill) => buildDocumentWithFonts({ markdown, resources }, withFill(fill),
  kitFonts(FONTS));
const docs = { off: await build(false), on: await build(true) };

const DEPTH = 150; // mm: how far down the page the painting bleeds
const sans = { fontFamily: 'Public Sans', fontWeight: 700, textTransform: 'uppercase' };
const at = (to, edge, x, y, width) => ({ anchor: { to, edge },
  offset: { x: mm(x), y: mm(y) }, ...(width && { size: { width: mm(width) } }) });
const air = { padding: { bottom: mm(6) } }; // counts: the text starts 6 mm under the byline
const opener = {
  enabled: true,
  slot: { elements: [
    { kind: 'image', id: 'art', resourceId: 'lens', placement: { anchor: { to: 'bleed',
      edge: 'top-left' }, size: { width: 'fill', height: mm(DEPTH) } } },
    { kind: 'text', id: 'credit', content: '{attr.credit}', ...sans, fontWeight: 500,
      fontSize: pt(6.5), letterSpacing: pt(0.6), color: col('muted'), align: 'right',
      placement: at('container', 'top-right', 0, DEPTH - TOP + 2) },
    { kind: 'text', id: 'kicker', content: '{attr.kicker}', ...sans, fontSize: pt(8.5),
      letterSpacing: pt(1.7), color: col('brass'), align: 'left',
      placement: at('container', 'top-left', 0, DEPTH - TOP + 9) },
    { kind: 'text', id: 'headline', content: '{titleText}', fontFamily: 'Libre Caslon Display',
      fontSize: pt(46), lineHeight: 1.02, color: col('ink'), align: 'left',
      placement: at('#kicker', 'below', 0, 2.5, 112) },
    { kind: 'text', id: 'standfirst', content: '{attr.standfirst}', fontFamily: 'Libre Caslon Text',
      italic: true, fontSize: pt(12), lineHeight: 1.3, color: col('sea'), align: 'left',
      placement: at('#headline', 'right-of', 8, 2.5) },
    { kind: 'text', id: 'byline', content: '{attr.byline}', ...sans, fontSize: pt(7.5),
      letterSpacing: pt(1.3), color: col('ink'), align: 'left', box: air,
      placement: at('#standfirst', 'below', 0, 3.5) },
  ] },
};

const label = { ...sans, fontWeight: 600, fontSize: pt(7.5), letterSpacing: pt(1.3),
  color: col('muted') };
const folio = { fontFamily: 'Public Sans', fontWeight: 700, fontSize: pt(8.5), color: col('ink') };
const pin = (edge, x, y) => ({ anchor: { to: 'page', edge }, offset: { x: mm(x), y: mm(y) } });
const sides = [['even', 'left', 1], ['odd', 'right', -1]]; // 1: the outer edge is on the left
const header = { elements: sides.flatMap(([parity, edge, s]) => [
  { kind: 'text', id: `folio-${parity}`, content: '{pageNumber}', ...folio,
    placement: pin(`top-${edge}`, s * OUTER, 12) },
  { kind: 'text', id: `head-${parity}`, ...label,
    content: s > 0 ? '{title} · {subtitle}' : '{chapterTitle}',
    placement: pin(`top-${edge}`, s * (OUTER + 9), 12.35) }, // the smaller size's baseline
  { kind: 'rule', id: `rule-${parity}`, direction: 'horizontal', thickness: pt(0.5),
    color: col('rule'), placement: { ...pin(`top-${edge}`, s * OUTER, 17),
      size: { width: mm(197) } } },
].map((element) => ({ ...element, parity, pages: 'body' }))) }; // none on the opener
const footer = { elements: sides.map(([parity, edge, s]) => ({ kind: 'text', parity,
  id: `drop-folio-${parity}`, ...folio, content: '{pageNumber}', pages: 'opener', align: edge,
  placement: pin(`bottom-${edge}`, s * OUTER, -11) })) }; // the opener's only folio