fix(nav): scroll-restore key auf nav.from.url, nicht location

Live-Test auf kochwas-dev offenbarte: bei Browser-Back hat der Browser
die History bereits gepoppt, bevor SvelteKit beforeNavigate feuert —
location.pathname war damit schon die Ziel-URL. recordScroll() schrieb
also den 0-Wert der Recipe-Page in den Slot der Home-Page und wischte
den eigentlich gemerkten Wert (z. B. 500) raus. Restore las dann 0,
fiel unter MIN_RESTORE_Y und tat nichts.

Fix: recordScroll(nav.from?.url) und restoreScroll(type, nav.to?.url).
Helper bekommen die URL explizit reingereicht — keine Abhängigkeit
mehr von location und damit kein Race mit der Browser-History.

Tests: zusätzliche Regression "does not overwrite a stored URL when
called with a different from-url" plus Skip-Pfade fuer null URLs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/scroll-restore.test.ts

@@ -20,6 +20,10 @@ function setViewportHeight(h: number) {
   Object.defineProperty(window, 'innerHeight', { value: h, configurable: true });
 }
 
+function url(path: string): URL {
+  return new URL(path, 'https://example.test');
+}
+
 describe('scroll-restore', () => {
   let scrollToSpy: ReturnType<typeof vi.spyOn>;
 
@@ -31,7 +35,6 @@ describe('scroll-restore', () => {
     scrollToSpy = vi
       .spyOn(window, 'scrollTo')
       .mockImplementation(() => undefined as unknown as void);
-    window.history.replaceState({}, '', '/wishlist');
   });
 
   afterEach(() => {
@@ -39,64 +42,84 @@ describe('scroll-restore', () => {
     vi.useRealTimers();
   });
 
-  it('records scrollY keyed by pathname+search', () => {
+  it('records scrollY keyed by from-url pathname+search', () => {
     setScrollY(1200);
-    recordScroll();
+    recordScroll(url('/wishlist'));
     const map = JSON.parse(sessionStorage.getItem(STORAGE_KEY) ?? '{}');
     expect(map['/wishlist']).toBe(1200);
   });
 
   it('keeps separate entries per URL', () => {
     setScrollY(500);
-    recordScroll();
-    window.history.replaceState({}, '', '/?q=hi');
+    recordScroll(url('/wishlist'));
     setScrollY(900);
-    recordScroll();
+    recordScroll(url('/?q=hi'));
     const map = JSON.parse(sessionStorage.getItem(STORAGE_KEY) ?? '{}');
     expect(map['/wishlist']).toBe(500);
     expect(map['/?q=hi']).toBe(900);
   });
 
+  it('does not overwrite a stored URL when called with a different from-url', () => {
+    // This is the regression: on popstate, location.pathname is already
+    // the new URL. Recording must use nav.from.url (the page being left),
+    // not location, or we wipe the destination's saved scrollY.
+    setScrollY(500);
+    recordScroll(url('/'));
+    setScrollY(0);
+    recordScroll(url('/recipes/1'));
+    const map = JSON.parse(sessionStorage.getItem(STORAGE_KEY) ?? '{}');
+    expect(map['/']).toBe(500);
+    expect(map['/recipes/1']).toBe(0);
+  });
+
+  it('skips when from-url is missing', () => {
+    setScrollY(900);
+    recordScroll(null);
+    expect(sessionStorage.getItem(STORAGE_KEY)).toBeNull();
+  });
+
   it('skips restore for non-popstate navigation', () => {
     sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1000 }));
     setDocHeight(5000);
-    restoreScroll('link');
+    restoreScroll('link', url('/wishlist'));
+    expect(scrollToSpy).not.toHaveBeenCalled();
+  });
+
+  it('skips restore when to-url is missing', () => {
+    sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1000 }));
+    restoreScroll('popstate', null);
     expect(scrollToSpy).not.toHaveBeenCalled();
   });
 
   it('skips restore when no entry stored', () => {
     setDocHeight(5000);
-    restoreScroll('popstate');
+    restoreScroll('popstate', url('/wishlist'));
     expect(scrollToSpy).not.toHaveBeenCalled();
   });
 
   it('skips restore for trivial scrollY (noise)', () => {
     sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 10 }));
     setDocHeight(5000);
-    restoreScroll('popstate');
+    restoreScroll('popstate', url('/wishlist'));
     expect(scrollToSpy).not.toHaveBeenCalled();
   });
 
   it('scrolls immediately when document is already tall enough', async () => {
     sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1000 }));
     setDocHeight(5000);
-    restoreScroll('popstate');
-    // rAF resolves on next microtask in jsdom
+    restoreScroll('popstate', url('/wishlist'));
     await new Promise((r) => requestAnimationFrame(() => r(null)));
     expect(scrollToSpy).toHaveBeenCalledWith({ top: 1000, left: 0, behavior: 'instant' });
   });
 
   it('waits via rAF until document grows tall enough', async () => {
     sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1500 }));
-    // Initially short — would clamp.
     setDocHeight(900);
-    restoreScroll('popstate');
+    restoreScroll('popstate', url('/wishlist'));
     await new Promise((r) => requestAnimationFrame(() => r(null)));
     expect(scrollToSpy).not.toHaveBeenCalled();
 
-    // Simulate async data loading and document growing.
     setDocHeight(3000);
-    // Allow rAF to fire again.
     await new Promise((r) => requestAnimationFrame(() => r(null)));
     expect(scrollToSpy).toHaveBeenCalledWith({ top: 1500, left: 0, behavior: 'instant' });
   });