From ed7cfc31fc242f91c6e40a523b920c1d8ef724ab Mon Sep 17 00:00:00 2001
From: Huang Xin <chrox.huang@gmail.com>
Date: Fri, 10 Apr 2026 02:08:55 +0800
Subject: [PATCH] fix(layout): fix off-by-one page count on fractional DPR
 devices, closes #3787 (#3813)
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The `pages` getter used Math.ceil(viewSize / containerSize) which inflates
the count by 1 when floating-point drift makes the ratio slightly above an
integer (e.g. 4.00000001 → 5). Use Math.round to absorb sub-pixel drift,
matching the approach already used in #getPagesBeforeView.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 .../document/paginator-multiview.test.ts      | 34 +++++++++++++++++++
 packages/foliate-js                           |  2 +-
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/apps/readest-app/src/__tests__/document/paginator-multiview.test.ts b/apps/readest-app/src/__tests__/document/paginator-multiview.test.ts
index bbff5a15..9d1eb2e5 100644
--- a/apps/readest-app/src/__tests__/document/paginator-multiview.test.ts
+++ b/apps/readest-app/src/__tests__/document/paginator-multiview.test.ts
@@ -278,6 +278,40 @@ describe('Paginator multi-view architecture', () => {
       // With tolerance, this should detect index 3 (not 2)
       expect(detectPrimary(visibleStart)).toBe(3);
     });
+
+    it('should not inflate page count due to fractional DPR rounding (pages getter)', () => {
+      // On fractional DPR devices, getBoundingClientRect() on the view element
+      // returns a width that is a near-integer multiple of the container size,
+      // but with tiny floating-point drift. Math.ceil inflates the count by 1.
+      // Real scenario: containerSize=785.45458984375, viewSize should be
+      // exactly 4*containerSize but getBoundingClientRect() returns a value
+      // with sub-pixel error.
+      const containerSize = 785.45458984375;
+      const viewSize = containerSize * 4 + 0.0001; // tiny FP drift
+
+      // Bug: Math.ceil gives 5 instead of 4
+      const buggyPages = Math.ceil(viewSize / containerSize);
+      expect(buggyPages).toBe(5); // confirms the bug exists
+
+      // Fix: Math.round absorbs sub-pixel drift
+      const fixedPages = Math.round(viewSize / containerSize);
+      expect(fixedPages).toBe(4);
+    });
+
+    it('should not inflate rendered page count due to fractional DPR rounding', () => {
+      // Same issue for #renderedPages which sums multiple view sizes
+      const containerSize = 785.45458984375;
+      const viewSizes = [containerSize * 3 + 0.00005, containerSize * 2 + 0.00008];
+      const totalViewSize = viewSizes.reduce((a, b) => a + b, 0);
+
+      // Bug: Math.ceil over-counts
+      const buggyPages = Math.ceil(totalViewSize / containerSize);
+      expect(buggyPages).toBe(6); // should be 5 but ceil rounds up
+
+      // Fix: Math.round
+      const fixedPages = Math.round(totalViewSize / containerSize);
+      expect(fixedPages).toBe(5);
+    });
   });
 
   describe('Adjacent index with fromIndex parameter', () => {
diff --git a/packages/foliate-js b/packages/foliate-js
index fe33bb51..d3baa52d 160000
--- a/packages/foliate-js
+++ b/packages/foliate-js
@@ -1 +1 @@
-Subproject commit fe33bb510871bd0489493c49638a5b82c1de5df5
+Subproject commit d3baa52d05047b2aa02779294f02a81fac855188