diff --git a/apps/readest-app/src/__tests__/document/fixed-layout-spine-seam.test.ts b/apps/readest-app/src/__tests__/document/fixed-layout-spine-seam.test.ts
new file mode 100644
index 00000000..6fb12378
--- /dev/null
+++ b/apps/readest-app/src/__tests__/document/fixed-layout-spine-seam.test.ts
@@ -0,0 +1,55 @@
+// Regression test for readest/readest issue #4857.
+//
+// In a fixed-layout (EPUB or PDF) two-page spread at a fractional device pixel
+// ratio (Windows display scale 150% -> devicePixelRatio 1.5), a one-pixel white
+// seam appeared down the middle of the spread at the spine.
+//
+// Root cause: the two page iframes are independent compositor layers, each
+// scaled by a (usually non-integer) factor. At a fractional devicePixelRatio the
+// spine between them lands on a fractional device pixel, so each layer's edge
+// there is anti-aliased against transparency and the reader background bleeds
+// through as a thin white seam. (Filling the page canvas to its box, #4587, does
+// not help: the soft edge comes from scaling the layer, not from the content
+// stopping short of its box.)
+//
+// The fix overlaps the (top-most) right page onto the left by exactly one device
+// pixel, so each soft edge sits over the neighbour's opaque content instead of
+// the background. The shift is visual-only (translateX), leaving the centred
+// spread layout untouched. `computeSpreadSpineOverlap` returns that translateX
+// (in CSS px) for the right page, or 0 for layouts with no touching spine.
+
+import { describe, expect, it } from 'vitest';
+
+import { computeSpreadSpineOverlap } from 'foliate-js/fixed-layout.js';
+
+describe('computeSpreadSpineOverlap (#4857)', () => {
+  it('overlaps the right page by exactly one device pixel on a real two-page spread', () => {
+    // Windows 150% scale: one device pixel is 1 / 1.5 CSS px.
+    expect(computeSpreadSpineOverlap({ devicePixelRatio: 1.5 })).toBeCloseTo(-1 / 1.5, 10);
+    // Retina / mobile: one device pixel is 0.5 CSS px.
+    expect(computeSpreadSpineOverlap({ devicePixelRatio: 2 })).toBeCloseTo(-0.5, 10);
+    // No scaling: one device pixel is one CSS px.
+    expect(computeSpreadSpineOverlap({ devicePixelRatio: 1 })).toBeCloseTo(-1, 10);
+  });
+
+  it('defaults a missing/zero devicePixelRatio to 1 device pixel', () => {
+    expect(computeSpreadSpineOverlap({})).toBeCloseTo(-1, 10);
+    expect(computeSpreadSpineOverlap({ devicePixelRatio: 0 })).toBeCloseTo(-1, 10);
+  });
+
+  it('does not overlap layouts that have no touching spine', () => {
+    // A single centred page (cover / odd page).
+    expect(computeSpreadSpineOverlap({ center: true, devicePixelRatio: 1.5 })).toBe(0);
+    // Portrait device: only one page of the spread is shown.
+    expect(computeSpreadSpineOverlap({ portrait: true, devicePixelRatio: 1.5 })).toBe(0);
+    // A spread slot padded with a blank page shows a single real page.
+    expect(computeSpreadSpineOverlap({ leftBlank: true, devicePixelRatio: 1.5 })).toBe(0);
+    expect(computeSpreadSpineOverlap({ rightBlank: true, devicePixelRatio: 1.5 })).toBe(0);
+  });
+
+  it('still overlaps a two-page spread when zoomed below 100%', () => {
+    // Zoom is intentionally not an input: the pages stay adjacent at every zoom,
+    // so a sub-100% spread gets the same overlap as a 100% one.
+    expect(computeSpreadSpineOverlap({ devicePixelRatio: 2 })).toBeCloseTo(-0.5, 10);
+  });
+});
diff --git a/packages/foliate-js b/packages/foliate-js
index 0fa407c4..0f857071 160000
--- a/packages/foliate-js
+++ b/packages/foliate-js
@@ -1 +1 @@
-Subproject commit 0fa407c4c63f09a5795c808503d1030231c645f0
+Subproject commit 0f85707129f61f42ee7a9313b7bd58a2afd0432d