Improve image preloading of previous/next chapter pages

The previous/next chapter only ever preloaded the first page, ignoring available preload contingent.
I.e., if all leading/trailing pages of the current chapter were loaded, the reader did not keep preloading the previous/next chapters pages

src/features/reader/Reader.types.ts

@@ -363,6 +363,7 @@ export interface ReaderPagerProps
     resumeMode: ReaderResumeMode;
     handleAsInitialRender: boolean;
     ref?: Ref<HTMLDivElement>;
+    currentChapterRemainingPages: number;
 }
 
 export enum PageInViewportType {

src/features/reader/viewer/ReaderChapterViewer.tsx

@@ -81,6 +81,7 @@ const BaseReaderChapterViewer = ({
     minWidth,
     minHeight,
     scrollElement,
+    currentChapterRemainingPages,
 }: Pick<ReaderStatePages, 'currentPageIndex' | 'transitionPageMode' | 'retryFailedPagesKeyPrefix'> &
     Omit<ReaderPagerProps, 'pages' | 'totalPages' | 'pageLoadStates' | 'handleAsInitialRender' | 'resumeMode'> &
     Pick<
@@ -433,6 +434,7 @@ const BaseReaderChapterViewer = ({
                 isPreloadMode={isPreloadMode}
                 resumeMode={resumeMode}
                 handleAsInitialRender={scrollIntoView}
+                currentChapterRemainingPages={currentChapterRemainingPages}
             />
             {showNextTransitionPage && (
                 <ReaderTransitionPage chapterId={chapterId} type={ReaderTransitionPageMode.NEXT} />

src/features/reader/viewer/ReaderViewer.tsx

@@ -51,6 +51,7 @@ import {
     useReaderSettingsStore,
 } from '@/features/reader/stores/ReaderStore.ts';
 import { STABLE_EMPTY_OBJECT } from '@/base/Base.constants.ts';
+import { getPage } from '@/features/reader/overlay/progress-bar/ReaderProgressBar.utils.tsx';
 
 const READING_MODE_TO_IN_VIEWPORT_TYPE: Record<ReadingMode, PageInViewportType> = {
     [ReadingMode.SINGLE_PAGE]: PageInViewportType.X,
@@ -290,10 +291,13 @@ const BaseReaderViewer = ({
                 const isCurrentChapter = chapter.id === currentChapter.id;
                 const isPreviousChapter = chapter.id === chaptersToRender[currentChapterIndex + 1]?.id;
                 const isNextChapter = chapter.id === chaptersToRender[currentChapterIndex - 1]?.id;
+                const isAdjacentChapterToCurrentChapter = isPreviousChapter || isNextChapter;
+
                 const isLeadingChapter = initialChapter.sourceOrder > chapter.sourceOrder;
                 const isTrailingChapter = initialChapter.sourceOrder < chapter.sourceOrder;
                 const isLastLeadingChapter = visibleChapters.lastLeadingChapterSourceOrder === chapter.sourceOrder;
                 const isLastTrailingChapter = visibleChapters.lastTrailingChapterSourceOrder === chapter.sourceOrder;
+
                 const isPreloadMode =
                     (isLastLeadingChapter && visibleChapters.isLeadingChapterPreloadMode) ||
                     (isLastTrailingChapter && visibleChapters.isTrailingChapterPreloadMode);
@@ -306,6 +310,29 @@ const BaseReaderViewer = ({
 
                 const isChapterSizeSourceChapter = chapter.id === minChapterSizeSourceChapterId;
 
+                const currentChapterCurrentPageIndex = isAdjacentChapterToCurrentChapter
+                    ? getReaderChapterViewerCurrentPageIndex(
+                          currentPageIndex,
+                          currentChapter,
+                          currentChapter,
+                          true,
+                          isCurrentChapterReady,
+                          initialChapter.sourceOrder > currentChapter.sourceOrder,
+                          initialChapter.sourceOrder < currentChapter.sourceOrder,
+                          visibleChapters,
+                      )
+                    : -1;
+                const currentChapterCurrentPagesIndex = isAdjacentChapterToCurrentChapter
+                    ? getPage(currentChapterCurrentPageIndex, pages).pagesIndex
+                    : -1;
+
+                const currentChapterRemainingLeadingPages = currentChapterCurrentPagesIndex;
+                const currentChapterRemainingTrailingPages = pages.length - 1 - currentChapterCurrentPagesIndex;
+
+                const currentChapterRemainingPages = isNextChapter
+                    ? currentChapterRemainingTrailingPages
+                    : currentChapterRemainingLeadingPages;
+
                 return (
                     <ReaderChapterViewer
                         key={chapter.id}
@@ -315,6 +342,7 @@ const BaseReaderViewer = ({
                         isPreviousChapterVisible={previousNextChapterVisibility.previous}
                         isNextChapterVisible={previousNextChapterVisibility.next}
                         lastPageRead={coerceIn(chapter.lastPageRead, 0, chapter.pageCount - 1)}
+                        currentChapterRemainingPages={currentChapterRemainingPages}
                         currentPageIndex={getReaderChapterViewerCurrentPageIndex(
                             currentPageIndex,
                             chapter,

src/features/reader/viewer/pager/ReaderPager.utils.tsx

@@ -533,12 +533,12 @@ export const isSpreadPage = (image: HTMLImageElement): boolean => {
 };
 
 const MIN_PREVIOUS_NEXT_CHAPTER_IMAGE_LOAD_AMOUNT = 0;
-const MAX_PREVIOUS_NEXT_CHAPTER_IMAGE_LOAD_AMOUNT = 1;
 const getImagePreLoadAmount = (
     isCurrentChapter: boolean,
     isPreviousChapter: boolean,
     isNextChapter: boolean,
     imagePreLoadAmount: number,
+    currentChapterRemainingPages: number,
 ): number => {
     if (isCurrentChapter) {
         return imagePreLoadAmount;
@@ -546,7 +546,7 @@ const getImagePreLoadAmount = (
 
     if (isPreviousChapter || isNextChapter) {
         return coerceIn(
-            MAX_PREVIOUS_NEXT_CHAPTER_IMAGE_LOAD_AMOUNT,
+            imagePreLoadAmount - currentChapterRemainingPages,
             MIN_PREVIOUS_NEXT_CHAPTER_IMAGE_LOAD_AMOUNT,
             imagePreLoadAmount,
         );
@@ -557,6 +557,7 @@ const getImagePreLoadAmount = (
 
 const PREVIOUS_IMAGE_LOAD_AMOUNT = 2;
 export const getPageIndexesToLoad = (
+    currentChapterRemainingPages: number,
     currentPageIndex: number,
     pages: ReaderStatePages['pages'],
     previousCurrentPageIndex: number,
@@ -576,6 +577,7 @@ export const getPageIndexesToLoad = (
         isPreviousChapter,
         isNextChapter,
         imagePreLoadAmount,
+        currentChapterRemainingPages,
     );
 
     const directionInvert = previousCurrentPageIndex <= currentPageIndex && !isPreviousChapter ? 1 : -1;

src/features/reader/viewer/pager/components/BasePager.tsx

@@ -40,6 +40,7 @@ const BaseBasePager = ({
     resumeMode,
     handleAsInitialRender,
     ref,
+    currentChapterRemainingPages,
 }: Omit<ReaderPagerProps, 'pageLoadStates' | 'retryFailedPagesKeyPrefix' | 'isPreloadMode'> &
     Pick<IReaderSettings, 'readingMode' | 'imagePreLoadAmount'> & {
         createPage: (
@@ -67,6 +68,7 @@ const BaseBasePager = ({
     const pagesIndexesToRender = useMemo(
         () =>
             getPageIndexesToLoad(
+                currentChapterRemainingPages,
                 currentPageIndex,
                 pages,
                 previousCurrentPageIndex.current,
@@ -76,7 +78,16 @@ const BaseBasePager = ({
                 isPreviousChapter,
                 isNextChapter,
             ),
-        [currentPageIndex, pages, imagePreLoadAmount, readingMode, isCurrentChapter, isPreviousChapter, isNextChapter],
+        [
+            currentChapterRemainingPages,
+            currentPageIndex,
+            pages,
+            imagePreLoadAmount,
+            readingMode,
+            isCurrentChapter,
+            isPreviousChapter,
+            isNextChapter,
+        ],
     );
     useEffect(() => {
         if (isCurrentChapter) {