fix: re-land explicit navigation when the reading stage changes mid-journey

src/journey/navigation-controller.ts

@@ -113,6 +113,13 @@ const browserRuntime = (): NavigationRuntime => ({
     });
   },
   focusHeading: (target) => {
+    // The live reading stage can start or stop while a long journey is still
+    // in flight (the scene boots lazily and steps down on slow GPUs), which
+    // shifts layout under the scroll. Re-land once, instantly, before focus.
+    const margin = parseFloat(getComputedStyle(target).scrollMarginTop) || 0;
+    const delta = target.getBoundingClientRect().top - margin;
+    if (Math.abs(delta) > 2)
+      window.scrollTo({ top: window.scrollY + delta, behavior: "instant" });
     target
       .querySelector<HTMLElement>('h1[tabindex="-1"], h2[tabindex="-1"]')
       ?.focus({ preventScroll: true });