Function renderFrameSlice

src/ink/log-update.ts:527–623

* Render a slice of rows from the frame's screen. * Each row is rendered followed by a newline. Cursor ends at (0, endY).

(
  screen: VirtualScreen,
  frame: Frame,
  startY: number,
  endY: number,
  stylePool: StylePool,
)

 * Each row is rendered followed by a newline. Cursor ends at (0, endY).
 */
function renderFrameSlice(
  screen: VirtualScreen,
  frame: Frame,
  startY: number,
  endY: number,
  stylePool: StylePool,
): VirtualScreen {
  let currentStyleId = stylePool.none
  let currentHyperlink: Hyperlink = undefined
  // Track the styleId of the last rendered cell on this line (-1 if none).
  // Passed to visibleCellAtIndex to enable fg-only space optimization.
  let lastRenderedStyleId = -1

  const { width: screenWidth, cells, charPool, hyperlinkPool } = frame.screen

  let index = startY * screenWidth
  for (let y = startY; y < endY; y += 1) {
    // Advance cursor to this row using LF (not CSI CUD / cursor-down).
    // CSI CUD stops at the viewport bottom margin and cannot scroll,
    // but LF scrolls the viewport to create new lines. Without this,
    // when the cursor is at the viewport bottom, moveCursorTo's
    // cursor-down silently fails, creating a permanent off-by-one
    // between the virtual cursor and the real terminal cursor.
    if (screen.cursor.y < y) {
      const rowsToAdvance = y - screen.cursor.y
      screen.txn(prev => {
        const patches: Diff = new Array<Diff[number]>(1 + rowsToAdvance)
        patches[0] = CARRIAGE_RETURN
        for (let i = 0; i < rowsToAdvance; i++) {
          patches[1 + i] = NEWLINE
        }
        return [patches, { dx: -prev.x, dy: rowsToAdvance }]
      })
    }
    // Reset at start of each line — no cell rendered yet
    lastRenderedStyleId = -1

    for (let x = 0; x < screenWidth; x += 1, index += 1) {
      // Skip spacers, unstyled empty cells, and fg-only styled spaces that
      // match the last rendered style (since cursor-forward produces identical
      // visual result). visibleCellAtIndex handles the optimization internally
      // to avoid allocating Cell objects for skipped cells.
      const cell = visibleCellAtIndex(
        cells,
        charPool,
        hyperlinkPool,
        index,
        lastRenderedStyleId,
      )
      if (!cell) {
        continue
      }

      moveCursorTo(screen, x, y)

      // Handle hyperlink
      const targetHyperlink = cell.hyperlink
      currentHyperlink = transitionHyperlink(