fix(app-router): respect scroll padding top as viewport boundary during navigation

[NriotHrreion]

Closes #2810. Ports vercel/next.js#96308 (vercel/next.js@da90782)

Overview

Align App Router navigation scrolling with Next.js when applications reserve space for sticky headers using root-level scroll-padding-top.

This ensures navigation targets that are visually obscured within the reserved top offset are treated as outside the usable viewport, while targets already visible below that boundary retain the current scroll position.

What changed

Added a function getScrollPaddingTopPx() ported from Next.js.

function getScrollPaddingTopPx(element: HTMLElement, viewportHeight: number): number {
  const scrollPaddingTop = getComputedStyle(element).scrollPaddingTop;
  const value = Number.parseFloat(scrollPaddingTop);
  if (!Number.isFinite(value) || value < 0) return 0;
  if (scrollPaddingTop.endsWith("px")) return value;
  if (scrollPaddingTop.endsWith("%")) return (value / 100) * viewportHeight;
  return 0;
}

And this function is lazily called inside topOfElementInViewport() to ensure getComputedStyle() is called after checking rect !== null. (And this is also how Next.js does)

Verification

• playwright test --project=app-router tests/e2e/app-router/nextjs-compat/router-autoscroll.spec.ts

This fix has been verified in my local repro project.

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commit: be254b9

[github-actions]

Performance benchmarks

Compared be254b9 against base 07cf9e5 using alternating same-runner rounds. Next.js was unchanged and skipped.

1 improved · 0 regressed · 5 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	134.7 KB	134.7 KB	⚫ +0.1%
Client entry size (gzip)	vinext	122.2 KB	122.3 KB	⚫ +0.1%
Dev server cold start	vinext	2.94 s	2.87 s	🟢 -2.4%
Production build time	vinext	3.16 s	3.16 s	⚫ -0.2%
RSC entry closure size (gzip)	vinext	113.9 KB	113.8 KB	⚫ -0.0%
Server bundle size (gzip)	vinext	191.4 KB	191.5 KB	⚫ +0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}