fix(ui): scroll-coupled backdrop pan that works in every browser

CSS animation-timeline: scroll() is Chromium-only, so the backdrop didn't move
for non-Chromium browsers. Replace it with a tiny rAF-throttled scroll handler
in site.js (the existing same-origin script — CSP stays strict, no inline JS)
that drives a --bg-shift CSS var off scroll position. The board now pans
vertically in lock-step with scroll and sits still at rest, in all browsers.

public/site.js

@@ -55,5 +55,37 @@
         io.observe(el);
       });
     }
+
+    // --- Scroll-coupled background pan -------------------------------------
+    // The Bat-Computer backdrop pans vertically in lock-step with scroll —
+    // moving ONLY while you scroll, still at rest. Plain scroll listener so it
+    // works in every browser (unlike CSS animation-timeline: scroll(), which is
+    // Chromium-only). rAF-throttled; sets a CSS var the transform reads.
+    var bgImgs = document.querySelectorAll(".site-bg__img");
+    if (bgImgs.length) {
+      var ticking = false;
+      function updateBg() {
+        ticking = false;
+        var doc = document.documentElement;
+        var max = doc.scrollHeight - window.innerHeight;
+        var p = max > 0 ? Math.min(1, Math.max(0, window.scrollY / max)) : 0;
+        // +half a range at the top → -half at the bottom: scrolling down pans
+        // the camera down the board. Range kept within the image's overhang so
+        // an edge never shows (image is 215% tall → 57.5vh spare each side).
+        var shift = (0.5 - p) * window.innerHeight * 0.7;
+        for (var i = 0; i < bgImgs.length; i++) {
+          bgImgs[i].style.setProperty("--bg-shift", shift.toFixed(1) + "px");
+        }
+      }
+      function onScroll() {
+        if (!ticking) {
+          ticking = true;
+          window.requestAnimationFrame(updateBg);
+        }
+      }
+      window.addEventListener("scroll", onScroll, { passive: true });
+      window.addEventListener("resize", onScroll, { passive: true });
+      updateBg();
+    }
   });
 })();

src/styles/global.css

@@ -372,12 +372,15 @@ svg {
   top: 50%;
   left: 50%;
   /* Zoomed in — larger than viewport so the board reads big/immersive and the
-     scroll-pan has room to travel. */
+     scroll-pan has room to travel without ever revealing an edge. */
   width: 215%;
   height: 215%;
   object-fit: cover;
-  transform: translate(-50%, -50%);
+  /* --bg-shift is driven by the scroll handler in site.js (scroll-coupled pan:
+     moves only as the page scrolls). Defaults to 0 → centred when JS is off. */
+  transform: translate(-50%, calc(-50% + var(--bg-shift, 0px)));
   will-change: transform;
+  transition: transform 0.15s linear;
 }
 .site-bg__img--light {
   display: none;
@@ -420,43 +423,6 @@ svg {
       rgba(246, 248, 251, 0.82) 100%
     );
 }
-/* BASELINE motion (works in EVERY browser, no JS): a slow, continuous top↔bottom
-   drift. This is the guaranteed fallback — Firefox/Safari don't support
-   scroll-driven animations, so without this the board would sit dead-still. */
-@media (prefers-reduced-motion: no-preference) {
-  .site-bg__img {
-    animation: bgDrift 48s ease-in-out infinite alternate;
-  }
-}
-/* ENHANCEMENT: where scroll-driven animations exist (Chromium), couple the pan
-   to scroll position instead — overrides the timed drift on the same element. */
-@supports (animation-timeline: scroll()) {
-  @media (prefers-reduced-motion: no-preference) {
-    .site-bg__img {
-      animation: bgPan linear both;
-      animation-timeline: scroll(root block);
-    }
-  }
-}
-/* Continuous timed drift (fallback) — gentle, alternates so it never jumps. */
-@keyframes bgDrift {
-  from {
-    transform: translate(-50%, -38%);
-  }
-  to {
-    transform: translate(-50%, -62%);
-  }
-}
-/* Scroll-coupled monotonic top→bottom pan (Chromium): ~44% travel of the
-   215%-tall image, tied directly to page scroll progress. */
-@keyframes bgPan {
-  from {
-    transform: translate(-50%, -28%);
-  }
-  to {
-    transform: translate(-50%, -72%);
-  }
-}
 
 @media (prefers-reduced-motion: reduce) {
   .scroll-progress {