From 48b57c4a74319c8897d9570f3ce41c77284a56f3 Mon Sep 17 00:00:00 2001
From: schroda <50052685+schroda@users.noreply.github.com>
Date: Mon, 17 Nov 2025 02:11:05 +0100
Subject: [PATCH] Simplify reader visible page detection

---
 .../reader/services/ReaderControls.ts         | 16 +-----------
 .../reader/viewer/pager/ReaderPager.utils.tsx | 26 ++++++++++++++-----
 2 files changed, 21 insertions(+), 21 deletions(-)

diff --git a/src/features/reader/services/ReaderControls.ts b/src/features/reader/services/ReaderControls.ts
index 6b14ba8f..892bee16 100644
--- a/src/features/reader/services/ReaderControls.ts
+++ b/src/features/reader/services/ReaderControls.ts
@@ -494,22 +494,8 @@ export class ReaderControls {
         type: PageInViewportType,
         readingDirection: ReadingDirection,
     ) {
-        const themeDirectionForReadingDirection = READING_DIRECTION_TO_THEME_DIRECTION[readingDirection];
-
         const firstVisibleImageIndex = imageRefs.current.findIndex(
-            (image) =>
-                image &&
-                isPageInViewport(
-                    image,
-                    type,
-                    // the pages aren't always properly scrolled to the top which results in the leading page to still be slightly
-                    // visible (e.g. 0.0123px) breaking the "first visible image" detection
-                    {
-                        bottom: 1,
-                        left: getOptionForDirection(0, 1, themeDirectionForReadingDirection),
-                        right: getOptionForDirection(1, 0, themeDirectionForReadingDirection),
-                    },
-                ),
+            (image) => image && isPageInViewport(image, type, { truncateValues: true }),
         );
         const lastPage = imageRefs.current?.[imageRefs.current.length - 1];
         const isEndReached = lastPage && isEndOfPageInViewport(lastPage, type, readingDirection);
diff --git a/src/features/reader/viewer/pager/ReaderPager.utils.tsx b/src/features/reader/viewer/pager/ReaderPager.utils.tsx
index 6bc91553..e4f16bd1 100644
--- a/src/features/reader/viewer/pager/ReaderPager.utils.tsx
+++ b/src/features/reader/viewer/pager/ReaderPager.utils.tsx
@@ -333,13 +333,21 @@ type InViewportThresholds = {
     left?: number;
     right?: number;
 };
-const getIsPageInViewportInfo = (
-    element: HTMLElement,
+type InViewportOptions = {
+    /**
+     * The pages aren't always properly scrolled to the start/end (e.g., via scrollIntoView) which results in a side to be slightly
+     * visible (e.g. 0.0123px) breaking the visibility detection
+     */
+    truncateValues?: boolean;
     /**
      * Thresholds are not considered for the detection of an image filling the whole viewport.
      * They are only used for detecting if a specific side of an image is inside the viewport
      */
-    argThresholds?: InViewportThresholds,
+    thresholds?: InViewportThresholds;
+};
+const getIsPageInViewportInfo = (
+    element: HTMLElement,
+    { truncateValues, thresholds: argThresholds }: InViewportOptions = { truncateValues: false, thresholds: {} },
 ): {
     isLeftInViewport: boolean;
     isRightInViewport: boolean;
@@ -348,7 +356,13 @@ const getIsPageInViewportInfo = (
     isBottomInViewport: boolean;
     isFillingHeightViewportCompletely: boolean;
 } => {
-    const { top, bottom, left, right } = element.getBoundingClientRect();
+    const { top: topRaw, bottom: bottomRaw, left: leftRaw, right: rightRaw } = element.getBoundingClientRect();
+
+    const maybeTruncateValue = (value: number): number => (truncateValues ? Math.trunc(value) : value);
+    const top = maybeTruncateValue(topRaw);
+    const bottom = maybeTruncateValue(bottomRaw);
+    const left = maybeTruncateValue(leftRaw);
+    const right = maybeTruncateValue(rightRaw);
 
     const thresholds = {
         top: 0,
@@ -379,7 +393,7 @@ const getIsPageInViewportInfo = (
 export const isPageInViewport = (
     element: HTMLElement,
     type: PageInViewportType,
-    thresholds?: InViewportThresholds,
+    options?: InViewportOptions,
 ): boolean => {
     const {
         isLeftInViewport,
@@ -388,7 +402,7 @@ export const isPageInViewport = (
         isTopInViewport,
         isBottomInViewport,
         isFillingHeightViewportCompletely,
-    } = getIsPageInViewportInfo(element, thresholds);
+    } = getIsPageInViewportInfo(element, options);
 
     const isInViewportX = isLeftInViewport || isRightInViewport || isFillingWidthViewportCompletely;
     const isInViewportY = isTopInViewport || isBottomInViewport || isFillingHeightViewportCompletely;