Function writeCellWithStyleStr

src/ink/log-update.ts:638–691

* Write a cell with a pre-serialized style transition string (from * StylePool.transition). Inlines the txn logic to avoid closure/tuple/delta * allocations on every cell. * * Returns true if the cell was written, false if skipped (wide char at * viewport edge). Callers MUST gate currentStyleId

(
  screen: VirtualScreen,
  cell: Cell,
  styleStr: string,
)

 * terminal, and the next transition is computed from phantom state.
 */
function writeCellWithStyleStr(
  screen: VirtualScreen,
  cell: Cell,
  styleStr: string,
): boolean {
  const cellWidth = cell.width === CellWidth.Wide ? 2 : 1
  const px = screen.cursor.x
  const vw = screen.viewportWidth

  // Don't write wide chars that would cross the viewport edge.
  // Single-codepoint chars (CJK) at vw-2 are safe; multi-codepoint
  // graphemes (flags, ZWJ emoji) need stricter threshold.
  if (cellWidth === 2 && px < vw) {
    const threshold = cell.char.length > 2 ? vw : vw + 1
    if (px + 2 >= threshold) {
      return false
    }
  }

  const diff = screen.diff
  if (styleStr.length > 0) {
    diff.push({ type: 'styleStr', str: styleStr })
  }

  const needsCompensation = cellWidth === 2 && needsWidthCompensation(cell.char)

  // On terminals with old wcwidth tables, a compensated emoji only advances
  // the cursor 1 column, so the CHA below skips column x+1 without painting
  // it. Write a styled space there first — on correct terminals the emoji
  // glyph (width 2) overwrites it harmlessly; on old terminals it fills the
  // gap with the emoji's background. Also clears any stale content at x+1.
  // CHA is 1-based, so column px+1 (0-based) is CHA target px+2.
  if (needsCompensation && px + 1 < vw) {
    diff.push({ type: 'cursorTo', col: px + 2 })
    diff.push({ type: 'stdout', content: ' ' })
    diff.push({ type: 'cursorTo', col: px + 1 })
  }

  diff.push({ type: 'stdout', content: cell.char })

  // Force terminal cursor to correct column after the emoji.
  if (needsCompensation) {
    diff.push({ type: 'cursorTo', col: px + cellWidth + 1 })
  }

  // Update cursor — mutate in place to avoid Point allocation
  if (px >= vw) {
    screen.cursor.x = cellWidth
    screen.cursor.y++
  } else {
    screen.cursor.x = px + cellWidth
  }
  return true
}

function moveCursorTo(screen: VirtualScreen, targetX: number, targetY: number) {
  screen.txn(prev => {