Phase 2C: fix reel centering via window-relative card positioning

The transitioned track translateY collapsed to 0 in Chromium (percent-
translated zero-height track), leaving the active cover one stride low
per step. The track is now a fixed anchor at the reel centre; each card
positions itself at (index - activeIndex) * stride and animates its own
small `top`, so no coordinate ever exceeds +/-RENDER_RADIUS * stride.

Verified with the transition ON: centre offset 0 at every index in the
12- and 500-item fixtures, 5 cards in the DOM at scale, wheel gates,
dialog semantics, Tab containment, focus restore, and polite announce
all pass. Dev window.__preview*/__ui handles removed from App.svelte.

src/App.svelte

@@ -6,7 +6,7 @@
     import ArtworkViewer from "./components/ArtworkViewer.svelte";
     import LoginModal from "./components/LoginModal.svelte";
     import PreviewOverlay from "./components/PreviewOverlay.svelte";
-    import { previewUi, openPreviewFixture } from "./lib/preview/previewSession.svelte.js";
+    import { previewUi } from "./lib/preview/previewSession.svelte.js";
     import Home from "./views/Home.svelte";
     import Search from "./views/Search.svelte";
     import Library from "./views/Library.svelte";
@@ -43,15 +43,6 @@
     refreshLibrary();
     refreshPlaylists();
 
-    // Dev-only hook for the Phase 2 exit-gate check (500-item fixture in a
-    // plain browser) and for driving the modal without a Tauri backend.
-    if (import.meta.env.DEV && typeof window !== "undefined") {
-        window.__previewFixture = (n = 12) => openPreviewFixture(n);
-        window.__previewUi = previewUi; // debug: the app's own store instance
-        window.__ui = ui; // debug: known-good reactive store, same instance
-        window.__togglePv = () => (previewUi.open = !previewUi.open);
-        window.__toggleNp = () => (ui.nowPlaying = !ui.nowPlaying);
-    }
 </script>
 
 <div

src/components/PreviewOverlay.svelte

@@ -84,14 +84,10 @@
 
     // ------------------------------------------------------------- reel ----
     // Virtualized: only a window of cards around the active index renders,
-    // positioned arithmetically (uniform stride), so a 500-item manifest
+    // positioned WINDOW-RELATIVE (offset from the active index, not from
-    // costs the same as a 10-item one — the Phase 2 exit gate.
+    // manifest index 0), so a 500-item manifest costs the same as a 10-item
-    // All reel geometry is CSS-computed from a single `--cover` clamp and a
+    // one — the Phase 2 exit gate — and no coordinate ever exceeds
-    // `--gap`, so JS never carries a pixel size that could disagree with the
+    // ±RENDER_RADIUS × stride.
-    // rendered box (the bug that left stepped cards off-centre). JS supplies
-    // only integer indices; CSS turns them into pixels. Virtualization: only
-    // a window of cards around the active index is in the DOM, positioned by
-    // absolute index, so a 500-item manifest costs the same as a 10-item one.
     const RENDER_RADIUS = 4;
     const GAP = 30;
     const windowStart = $derived(Math.max(0, (activeIndex ?? 0) - RENDER_RADIUS));
@@ -107,7 +103,7 @@
     // predict), measured off the live reel. CSS calc with an inherited
     // unitless var proved unreliable here, so JS owns the arithmetic — but
     // from the real pixel value, which is what made the earlier viewport-guess
-    // version drift. Cards and the track slide share this one number.
+    // version drift.
     let reelEl = $state(null);
     let stride = $state(220);
     function measureStride() {
@@ -124,7 +120,17 @@
         ro.observe(reelEl);
         return () => ro.disconnect();
     });
-    const trackOffset = $derived((activeIndex ?? 0) * stride);
+    // Window-relative positioning: each card sits at (index − activeIndex) ×
+    // stride from the reel centre, so the ACTIVE card is always exactly at the
+    // centre with NO track translate. The slide is each card's own small `top`
+    // transitioning; coordinates never exceed ±RENDER_RADIUS × stride, which
+    // kills two bugs at once — the giant absolute tops at index 500, and the
+    // transitioned track translateY that Chromium landed at 0 (a transition on
+    // a percent-translated zero-height track dropped the Y component; see
+    // HANDOFF-PHASE2C.md for the measurements).
+    function cardOffset(index) {
+        return (index - (activeIndex ?? 0)) * stride;
+    }
 
     function availabilityOf(item) {
         return st?.itemRuntimeByKey[item.sessionKey]?.availability ?? item.initialAvailability;
@@ -534,7 +540,7 @@
                         {#if activeItem?.artwork}
                             <img class="cover-glow" src={activeItem.artwork} alt="" aria-hidden="true" />
                         {/if}
-                        <div class="reel-track" style="transform: translateX(-50%) translateY({-trackOffset}px);">
+                        <div class="reel-track">
                             {#each visibleCards as { item, index } (item.sessionKey)}
                                 {@const avail = availabilityOf(item)}
                                 <div
@@ -542,7 +548,7 @@
                                     class:active={index === activeIndex}
                                     class:gone={avail !== "available"}
                                     class:hit={index === activeIndex && bumpKey > 0}
-                                    style="top: {index * stride}px; --idle-scale: {index === activeIndex ? 1 : 0.74};"
+                                    style="top: {cardOffset(index)}px; --idle-scale: {index === activeIndex ? 1 : 0.74};"
                                     onclick={() => avail === "available" && activate(item, index)}
                                     role={index === activeIndex ? undefined : "button"}
                                     aria-hidden={index !== activeIndex}
@@ -923,16 +929,16 @@
         mask-image: linear-gradient(180deg, transparent 0%, #000 18%, #000 82%, transparent 100%);
     }
     .reel-track {
+        /* A fixed anchor at the reel centre — it never moves. Cards position
+           themselves relative to it at (index − activeIndex) × stride and
+           animate their own `top`; a transitioned translateY on this track is
+           exactly what Chromium collapsed to 0 (see the file-top comment). */
         position: absolute;
         z-index: 1;
         left: 50%;
         top: 50%;
         width: var(--cover);
-        will-change: transform;
-        /* translateY is set inline from JS (measured stride × active index);
-           translateX(-50%) here centres the track's own width. */
         transform: translateX(-50%);
-        transition: transform 340ms cubic-bezier(0.26, 1.28, 0.6, 1);
     }
 
     /* Artwork bloom (B2): a blurred copy of the active cover, box-feathered
@@ -973,7 +979,9 @@
     .card {
         position: absolute;
         left: 0;
-        /* top set inline from JS (index × measured stride) */
+        /* top set inline from JS ((index − activeIndex) × measured stride);
+           transitioning it is what produces the slide — the track itself
+           never moves. */
         width: var(--cover);
         height: var(--cover);
         border-radius: 8px;
@@ -986,6 +994,7 @@
            box, so idle covers at 74% cannot perturb centring. */
         transform: translateY(-50%) scale(var(--idle-scale, 0.74));
         transition:
+            top 340ms cubic-bezier(0.26, 1.28, 0.6, 1),
             transform 340ms cubic-bezier(0.26, 1.28, 0.6, 1),
             opacity 0.3s ease,
             filter 0.3s ease,
@@ -1175,7 +1184,6 @@
 
     /* --------------------------------------------------- reduced motion --- */
     @media (prefers-reduced-motion: reduce) {
-        .reel-track { transition: none; }
         .card { transition: opacity 0.3s ease, filter 0.3s ease; }
         .card.active.hit { animation: none; }
         .modal.is-playing .cover-glow { animation: none; }