diff --git a/apps/readest-app/src/__tests__/document/fixed-layout-portrait-single-page.test.ts b/apps/readest-app/src/__tests__/document/fixed-layout-portrait-single-page.test.ts
new file mode 100644
index 00000000..17cb6c2d
--- /dev/null
+++ b/apps/readest-app/src/__tests__/document/fixed-layout-portrait-single-page.test.ts
@@ -0,0 +1,58 @@
+// Regression test for readest/readest issue #4984.
+//
+// In auto-spread mode a portrait viewport shows only one page of the two-page
+// spread. The renderer already hides the other page and scales the shown page
+// as a single page, but it kept the spread-centering inline margin: a left page
+// gets `margin-inline-start: auto` (which pushes it toward the spine on the
+// right) and a right page gets `margin-inline-end: auto` (pushing it left). With
+// only one page visible and no partner to meet, that one-sided auto margin
+// strands the page in one half of the viewport whenever it is narrower than the
+// viewport (any zoom below 100%, or a page whose fit-scaled width is less than
+// the viewport width). It also breaks tapping: the off-center page sits over a
+// page-turn tap zone instead of the center menu zone, so every tap turns the
+// page.
+//
+// The fix: in portrait, center the lone visible page (both inline margins auto)
+// instead of the one-sided margin. In landscape the two pages keep their
+// one-sided margins so they meet at the spine and the pair stays centered.
+// `computeSpreadInlineMargins(portrait)` returns the inline-margin style for the
+// left and right pages. It sets both inline margins explicitly so a re-render
+// after an orientation change fully overwrites the previous layout's margins
+// (frames are re-styled in place, not recreated, on rotation).
+
+import { describe, expect, it } from 'vitest';
+
+import { computeSpreadInlineMargins } from 'foliate-js/fixed-layout.js';
+
+describe('computeSpreadInlineMargins (#4984)', () => {
+  it('pushes each page toward the spine in landscape so the pair stays centered', () => {
+    const { left, right } = computeSpreadInlineMargins(false);
+    // Left page hugs the right (spine) edge; right page hugs the left edge.
+    expect(left.marginInlineStart).toBe('auto');
+    expect(left.marginInlineEnd).not.toBe('auto');
+    expect(right.marginInlineEnd).toBe('auto');
+    expect(right.marginInlineStart).not.toBe('auto');
+  });
+
+  it('centers the lone visible page in portrait instead of stranding it to a side', () => {
+    const { left, right } = computeSpreadInlineMargins(true);
+    // Whichever page of the spread is shown, its margins are symmetric so it is
+    // centered rather than shoved into one half of the viewport.
+    for (const style of [left, right]) {
+      expect(style.marginInlineStart).toBe('auto');
+      expect(style.marginInlineEnd).toBe('auto');
+    }
+  });
+
+  it('always sets both inline margins so an orientation change fully re-lays-out', () => {
+    // Both modes must define both sides; otherwise a stale `auto` left over from
+    // the previous orientation lingers and mis-centers the page.
+    for (const portrait of [false, true]) {
+      const { left, right } = computeSpreadInlineMargins(portrait);
+      for (const style of [left, right]) {
+        expect(style).toHaveProperty('marginInlineStart');
+        expect(style).toHaveProperty('marginInlineEnd');
+      }
+    }
+  });
+});
diff --git a/packages/foliate-js b/packages/foliate-js
index 7c9cc45b..f6dced2a 160000
--- a/packages/foliate-js
+++ b/packages/foliate-js
@@ -1 +1 @@
-Subproject commit 7c9cc45bafb8b56934b623186bc1c7dbb6a848a2
+Subproject commit f6dced2aa0ce21506029af771f66bc02a0bf14e3