fix(audio): Shift+grab opens the precision rail anchored at the playhead

The first grab implementation degraded Shift+grab to a quarter-speed
relative crawl — losing the ±5-minute rail the user expects when they
grab the handle with Shift held. The rail is back: Shift+grab anchors
railWindow at the CURRENT position (not the nearest chapter line, and
never teleports the playhead on pointerdown); plain grab stays relative
1:1. GRAB_FINE_SCALE is deleted as dead code, and the dialog guide
documents the restored gesture.

web/src/audio-keys.ts

@@ -233,7 +233,10 @@ export const MOUSE_GUIDE: readonly PointerGuideRow[] = [
     label: "Grab handle",
     description: "Drag the playhead from where it is — nothing moves until you do",
   },
-  { label: "Shift + grab", description: "Same handle drag at quarter speed for fine placement" },
+  {
+    label: "Shift + grab",
+    description: "Precision rail (±5 min) around the playhead's position",
+  },
   { label: "Shift + click", description: "Jump to the nearest chapter line" },
   { label: "Shift + drag", description: "Precision rail (±5 min) around the nearest line" },
   { label: "Wheel", description: "Nudge ±5s (Shift: ±60s)" },

web/src/audio-scrubber.test.ts

@@ -10,7 +10,6 @@ import {
   dragScale,
   DRAG_REF_PX,
   DRAG_SCALE_MIN,
-  GRAB_FINE_SCALE,
   GRAB_SLOP_PX,
   isHandleGrab,
   nearestChapterLine,
@@ -236,15 +235,13 @@ describe("isHandleGrab / applyGrab (§9 handle grab: no teleport)", () => {
     );
   });
 
-  it("crawls at fine scale without teleporting when Shift toggles mid-drag", () => {
+  it("moves 1:1 with pointer travel and clamps to the book", () => {
     const pxPerSecond = 800 / duration26h;
-    expect(applyGrab(5000, 100, pxPerSecond, duration26h, GRAB_FINE_SCALE)).toBeCloseTo(
+    expect(applyGrab(5000, 100, pxPerSecond, duration26h)).toBeCloseTo(
-      5000 + 100 * ((1 / pxPerSecond) * GRAB_FINE_SCALE),
+      5000 + (100 / 800) * duration26h,
       5,
     );
-    // Scale changes only affect SUBSEQUENT travel — a zero-delta move
+    expect(applyGrab(50, 100, 0, duration26h)).toBe(50);
-    // with a new scale must leave the position alone.
-    expect(applyGrab(5000, 0, pxPerSecond, duration26h, GRAB_FINE_SCALE)).toBe(5000);
   });
 
   it("clamps to the book and survives a dead bar", () => {

web/src/audio-scrubber.ts

@@ -79,7 +79,9 @@ export function snapToChapterPx(
 // thumb, in bar pixels, for it to count as grabbing the HANDLE rather
 // than clicking the track. A grab never teleports the position — not
 // to the click X (free-drag behavior) and not to a chapter line
-// (Shift+click behavior) — it drags RELATIVE to where the handle is.
+// (Shift+click behavior): a plain grab drags RELATIVE to where the
+// handle is, and Shift+grab opens the precision rail anchored at the
+// current position.
 export const GRAB_SLOP_PX = 12;
 
 // isHandleGrab reports whether a pointerdown at clientX lands on the
@@ -97,24 +99,19 @@ export function isHandleGrab(
   return Math.abs(clientX - thumbX) <= slopPx;
 }
 
-// GRAB_FINE_SCALE is the Shift+grab travel factor: the same relative
-// drag crawls at quarter speed for precise placement.
-export const GRAB_FINE_SCALE = 0.25;
-
 // applyGrab advances a HANDLE drag by one pointermove: incremental
-// (like applyDrag, a scale change never teleports the cursor), but
+// (like applyDrag, so the cursor never teleports), 1:1 with pointer
-// with a FIXED scale instead of a pull-away zoom — the mouse is always
+// travel. (Shift+grab uses the precision rail instead of a scaled
-// on the bar, so Shift is the precision control, not vertical offset.
+// drag — see scrubPointerDown.)
 export function applyGrab(
   position: number,
   dxPx: number,
   pxPerSecond: number,
   duration: number,
-  scale = 1,
 ): number {
   const clamped = Math.min(duration, Math.max(0, position));
   if (pxPerSecond <= 0 || duration <= 0) return clamped;
-  return Math.min(duration, Math.max(0, clamped + (dxPx / pxPerSecond) * scale));
+  return Math.min(duration, Math.max(0, clamped + dxPx / pxPerSecond));
 }
 
 // ---- §9 touch precision: vertical pull-away = progressive zoom ----

web/src/audio.ts

@@ -44,7 +44,6 @@ import {
   bufferedSpanPct,
   chapterIndexFor,
   denseTicks,
-  GRAB_FINE_SCALE,
   isHandleGrab,
   MAGNET_RADIUS_PX,
   nearestChapterLine,
@@ -235,11 +234,11 @@ Alpine.data("audioReader", () => ({
   railTo: 0,
   railTickList: [] as Array<{ start: number; title: string }>,
   // §9 interaction model: "free" = 1:1 mouse drag (no rail), "rail" =
-  // Shift+drag precision window, "touch" = incremental cursor with
+  // Shift+drag precision window (also Shift+grab ON the thumb, anchored
-  // pull-away precision, "grab" = the pointerdown landed ON the
+  // at the playhead), "touch" = incremental cursor with pull-away
-  // playhead thumb — the position drags RELATIVE from where it is
+  // precision, "grab" = the pointerdown landed ON the playhead thumb —
-  // (never teleporting to the click X or a chapter line; Shift =
+  // the position drags RELATIVE from where it is (never teleporting to
-  // quarter-speed fine crawl). The rail is only visible for the
+  // the click X or a chapter line). The rail is only visible for the
   // middle two.
   dragMode: null as null | "free" | "rail" | "touch" | "grab",
   railVisible: false,
@@ -945,12 +944,24 @@ Alpine.data("audioReader", () => ({
     } else if (isHandleGrab(e.clientX, rect, this.globalPosition(), duration)) {
       // Grabbing the HANDLE itself: the playhead must not teleport —
       // neither to the click X (free drag) nor to a chapter line
-      // (Shift+click). The thumb checks this BEFORE the Shift branch:
+      // (Shift+click). The thumb check runs BEFORE the bare Shift
-      // the grab is relative from where the handle is, and Shift only
+      // branch: plain grab drags relative from where the handle is;
-      // slows the same drag to a fine crawl.
+      // Shift+grab opens the precision rail ANCHORED AT THE CURRENT
-      this.dragMode = "grab";
+      // POSITION (±5 min around the playhead, not around the nearest
-      this.grabLastX = e.clientX;
+      // chapter line).
       this.scrubValue = Math.min(duration, Math.max(0, this.globalPosition()));
+      if (e.shiftKey) {
+        this.dragMode = "rail";
+        const window = railWindow(this.scrubValue, duration);
+        this.railFrom = window.from;
+        this.railTo = window.to;
+        this.railTickList = railTicks(window, this.chapters);
+        this.railVisible = true;
+        this.railLiftY = 0;
+      } else {
+        this.dragMode = "grab";
+        this.grabLastX = e.clientX;
+      }
     } else if (e.shiftKey) {
       // Shift+click/drag: the precision rail, anchored to the NEAREST
       // chapter line (Shift+click = snap regardless of radius).
@@ -998,16 +1009,9 @@ Alpine.data("audioReader", () => ({
     if (this.dragMode === "grab") {
       // Relative handle drag: each move advances from the CURRENT
       // position by the pointer's delta (never re-maps the click X).
-      // Shift = quarter-speed fine placement.
       const dx = e.clientX - this.grabLastX;
       this.grabLastX = e.clientX;
-      this.scrubValue = applyGrab(
+      this.scrubValue = applyGrab(this.scrubValue, dx, rect.width / duration, duration);
-        this.scrubValue,
-        dx,
-        rect.width / duration,
-        duration,
-        e.shiftKey ? GRAB_FINE_SCALE : 1,
-      );
       return;
     }
     if (this.dragMode === "touch") {