From dc88b42191454f679b897ed97251d8423236effc Mon Sep 17 00:00:00 2001
From: schroda <50052685+schroda@users.noreply.github.com>
Date: Sun, 23 Mar 2025 16:02:32 +0100
Subject: [PATCH] Fix preloading of reader pages

There were different issues with the direction the pages got preloaded.

- when resuming a chapter with an initial page > 1, it incorrectly preloaded leading pages instead of trailing ones.
- when going to the previous/next rendered chapter no pages got preloaded at all
---
 .../components/viewer/ReaderChapterViewer.tsx |  4 +++-
 .../components/viewer/pager/BasePager.tsx     | 19 +++++++++++++++++--
 src/modules/reader/types/Reader.types.ts      |  2 ++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/modules/reader/components/viewer/ReaderChapterViewer.tsx b/src/modules/reader/components/viewer/ReaderChapterViewer.tsx
index 232c6f79..01a21070 100644
--- a/src/modules/reader/components/viewer/ReaderChapterViewer.tsx
+++ b/src/modules/reader/components/viewer/ReaderChapterViewer.tsx
@@ -103,7 +103,7 @@ const BaseReaderChapterViewer = ({
     | 'retryFailedPagesKeyPrefix'
     | 'setTransitionPageMode'
 > &
-    Omit<ReaderPagerProps, 'pages' | 'totalPages' | 'pageLoadStates'> &
+    Omit<ReaderPagerProps, 'pages' | 'totalPages' | 'pageLoadStates' | 'handleAsInitialRender' | 'resumeMode'> &
     Pick<
         IReaderSettings,
         'readingMode' | 'shouldOffsetDoubleSpreads' | 'readingDirection' | 'readerWidth' | 'pageScaleMode'
@@ -450,6 +450,8 @@ const BaseReaderChapterViewer = ({
                 scrollbarYSize={scrollbarYSize}
                 readerNavBarWidth={readerNavBarWidth}
                 isPreloadMode={isPreloadMode}
+                resumeMode={resumeMode}
+                handleAsInitialRender={scrollIntoView}
             />
             {showNextTransitionPage && (
                 <ReaderTransitionPage chapterId={chapterId} type={ReaderTransitionPageMode.NEXT} />
diff --git a/src/modules/reader/components/viewer/pager/BasePager.tsx b/src/modules/reader/components/viewer/pager/BasePager.tsx
index 2b2454db..00c488e7 100644
--- a/src/modules/reader/components/viewer/pager/BasePager.tsx
+++ b/src/modules/reader/components/viewer/pager/BasePager.tsx
@@ -10,10 +10,18 @@ import { forwardRef, memo, ReactNode, useCallback, useEffect, useMemo, useRef }
 import Box, { BoxProps } from '@mui/material/Box';
 import { getPageIndexesToLoad, isATransitionPageVisible } from '@/modules/reader/utils/ReaderPager.utils.tsx';
 import { ReaderStatePages } from '@/modules/reader/types/ReaderProgressBar.types.ts';
-import { IReaderSettings, ReaderPagerProps, ReaderTransitionPageMode } from '@/modules/reader/types/Reader.types.ts';
+import {
+    IReaderSettings,
+    ReaderPagerProps,
+    ReaderResumeMode,
+    ReaderTransitionPageMode,
+} from '@/modules/reader/types/Reader.types.ts';
 import { applyStyles } from '@/modules/core/utils/ApplyStyles.ts';
 import { isContinuousReadingMode } from '@/modules/reader/utils/ReaderSettings.utils.tsx';
 
+const getPreviousCurrentPageIndex = (resumeMode: ReaderResumeMode): number =>
+    resumeMode === ReaderResumeMode.END ? Number.MAX_SAFE_INTEGER : -1;
+
 const BaseBasePager = forwardRef<
     HTMLDivElement,
     Omit<ReaderPagerProps, 'pageLoadStates' | 'retryFailedPagesKeyPrefix' | 'isPreloadMode'> &
@@ -56,10 +64,17 @@ const BaseBasePager = forwardRef<
             scrollbarXSize,
             scrollbarYSize,
             readerNavBarWidth,
+            resumeMode,
+            handleAsInitialRender,
         },
         ref,
     ) => {
-        const previousCurrentPageIndex = useRef(currentPageIndex > 0 ? Number.MAX_SAFE_INTEGER : -1);
+        const previousCurrentPageIndex = useRef(getPreviousCurrentPageIndex(resumeMode));
+
+        if (handleAsInitialRender) {
+            previousCurrentPageIndex.current = getPreviousCurrentPageIndex(resumeMode);
+        }
+
         const pagesIndexesToRender = useMemo(
             () =>
                 getPageIndexesToLoad(
diff --git a/src/modules/reader/types/Reader.types.ts b/src/modules/reader/types/Reader.types.ts
index cf8f56b9..37a50d37 100644
--- a/src/modules/reader/types/Reader.types.ts
+++ b/src/modules/reader/types/Reader.types.ts
@@ -292,6 +292,8 @@ export interface ReaderPagerProps
     isPreviousChapter: boolean;
     isNextChapter: boolean;
     isPreloadMode: boolean;
+    resumeMode: ReaderResumeMode;
+    handleAsInitialRender: boolean;
 }
 
 export enum PageInViewportType {