Function tailForViewport

src/cli/viewport.ts:22–48

(text: string, rows: number, cols: number)

 * wrapped height (ceil(len / cols)) until the row budget is spent.
 */
export function tailForViewport(text: string, rows: number, cols: number): string {
  const budget = Math.max(6, (rows || 24) - 10); // room for header, input box, status, margin
  const width = Math.max(20, cols || 80);
  const logical = text.split('\n');
  let used = 0;
  const kept: string[] = [];
  for (let i = logical.length - 1; i >= 0; i--) {
    const line = logical[i];
    const vis = Math.max(1, Math.ceil(line.length / width)); // wrapped rows for this line
    if (used + vis > budget) {
      // This line doesn't fully fit. A SINGLE line can wrap to more rows than the
      // whole budget (a long unbroken line, or wrapped RTL/Persian paragraph). If we
      // kept it whole it would push the live region past the terminal height, scroll
      // into scrollback, and — repainted each token — oscillate. So keep only the
      // TRAILING rows of this top line that still fit; its scrolled-off head is
      // exactly what the terminal would have hidden anyway. (Also covers the case
      // where nothing has been kept yet — the bottom line alone is taller than budget.)
      const rowsLeft = budget - used;
      if (rowsLeft > 0) kept.unshift(line.slice(line.length - rowsLeft * width));
      break;
    }
    used += vis;
    kept.unshift(line);
    if (used >= budget) break;
  }
  return kept.join('\n');
}

/**
 * Did the terminal get SMALLER in either dimension?