Function clipMainScreenFrameToVisibleRows

src/ink/log-update.ts:1119–1175

(
  frame: Frame,
  stylePool: StylePool,
)

}

function clipMainScreenFrameToVisibleRows(
  frame: Frame,
  stylePool: StylePool,
): Frame {
  // Main-screen repaint from CSI H starts at the visible viewport home, not at
  // the logical output origin in scrollback. renderFrameSlice emits CRLF after
  // every row, so painting a full viewport would scroll the bottom row and
  // duplicate/garble the visible screen. Keep the logical frame intact for the
  // next incremental diff, but repaint only the visible content suffix here.
  const visibleRows = Math.max(0, frame.viewport.height - 1)
  if (frame.screen.height <= visibleRows) {
    return frame
  }

  const sourceStartY = Math.max(0, frame.screen.height - visibleRows)
  const clippedHeight = frame.screen.height - sourceStartY
  const source = frame.screen
  const screen = createScreen(
    source.width,
    clippedHeight,
    stylePool,
    source.charPool,
    source.hyperlinkPool,
  )

  const cellWordsPerRow = source.width << 1
  const sourceCellStart = sourceStartY * cellWordsPerRow
  screen.cells.set(
    source.cells.subarray(
      sourceCellStart,
      sourceCellStart + clippedHeight * cellWordsPerRow,
    ),
  )

  const sourceNoSelectStart = sourceStartY * source.width
  screen.noSelect.set(
    source.noSelect.subarray(
      sourceNoSelectStart,
      sourceNoSelectStart + clippedHeight * source.width,
    ),
  )
  screen.softWrap.set(
    source.softWrap.subarray(sourceStartY, sourceStartY + clippedHeight),
  )
  screen.contentEnd.set(
    source.contentEnd.subarray(sourceStartY, sourceStartY + clippedHeight),
  )

  return {
    screen,
    viewport: frame.viewport,
    cursor: {
      ...frame.cursor,
      y: Math.max(0, frame.cursor.y - sourceStartY),
    },
  }
}