Method render

src/ink/log-update.ts:235–791

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

  }

  render(
    prev: Frame,
    next: Frame,
    altScreen = false,
    decstbmSafe = true,
  ): RenderOutput {
    const startTime = performance.now()
    const stylePool = this.options.stylePool
    const physicalNext = altScreen
      ? next
      : clipMainScreenFrameToVisibleRows(next, stylePool)

    if (!this.options.isTTY) {
      return {
        diff: this.renderFullFrame(next),
        physicalFrame: physicalNext,
      }
    }

    const widthResizeStaysIncremental =
	    prev.viewport.width !== next.viewport.width &&
	    canStayIncrementalAcrossWidthResize(prev, next)

    // 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 (shouldResetForResize(prev, next)) {
      this.state.highestIgnoredHiddenRow = undefined
      if (!altScreen && canRepaintFromPreviousOutputTop(prev)) {
        return {
          diff: fullRepaintFromPreviousOutputTop(prev, next, stylePool, {
            clearRowsBeforeWrite: true,
          }),
          physicalFrame: physicalNext,
        }
      }
      return {
        diff: fullViewportRepaintFromHome(next, stylePool, prev),
        physicalFrame: physicalNext,
      }
    }

    // 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