Method render

src/ink/log-update.ts:123–467

(
    prev: Frame,
    next: Frame,
    altScreen = false,
    decstbmSafe = true,
  )

  }

  render(
    prev: Frame,
    next: Frame,
    altScreen = false,
    decstbmSafe = true,
  ): Diff {
    if (!this.options.isTTY) {
      return this.renderFullFrame(next)
    }

    const startTime = performance.now()
    const stylePool = this.options.stylePool

    // Since we assume the cursor is at the bottom on the screen, we only need
    // to clear when the viewport gets shorter (i.e. the cursor position drifts)
    // or when it gets thinner (and text wraps). We _could_ figure out how to
    // not reset here but that would involve predicting the current layout
    // _after_ the viewport change which means calcuating text wrapping.
    // Resizing is a rare enough event that it's not practically a big issue.
    if (
      next.viewport.height < prev.viewport.height ||
      (prev.viewport.width !== 0 && next.viewport.width !== prev.viewport.width)
    ) {
      return fullResetSequence_CAUSES_FLICKER(next, 'resize', stylePool)
    }

    // DECSTBM scroll optimization: when a ScrollBox's scrollTop changed,
    // shift content with a hardware scroll (CSI top;bot r + CSI n S/T)
    // instead of rewriting the whole scroll region. The shiftRows on
    // prev.screen simulates the shift so the diff loop below naturally
    // finds only the rows that scrolled IN as diffs. prev.screen is
    // about to become backFrame (reused next render) so mutation is safe.
    // CURSOR_HOME after RESET_SCROLL_REGION is defensive — DECSTBM reset
    // homes cursor per spec but terminal implementations vary.
    //
    // decstbmSafe: caller passes false when the DECSTBM→diff sequence
    // can't be made atomic (no DEC 2026 / BSU/ESU). Without atomicity the
    // outer terminal renders the intermediate state — region scrolled,
    // edge rows not yet painted — a visible vertical jump on every frame
    // where scrollTop moves. Falling through to the diff loop writes all
    // shifted rows: more bytes, no intermediate state. next.screen from
    // render-node-to-output's blit+shift is correct either way.
    let scrollPatch: Diff = []
    if (altScreen && next.scrollHint && decstbmSafe) {
      const { top, bottom, delta } = next.scrollHint
      if (
        top >= 0 &&
        bottom < prev.screen.height &&
        bottom < next.screen.height
      ) {
        shiftRows(prev.screen, top, bottom, delta)
        scrollPatch = [
          {
            type: 'stdout',
            content:
              setScrollRegion(top + 1, bottom + 1) +
              (delta > 0 ? csiScrollUp(delta) : csiScrollDown(-delta)) +
              RESET_SCROLL_REGION +