Start preloading next chapter if within last 3 pages instead of last page

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerViewer.kt

@@ -144,8 +144,10 @@ abstract class PagerViewer(val activity: ReaderActivity) : BaseViewer {
         Timber.d("onReaderPageSelected: ${page.number}/${pages.size}")
         activity.onPageSelected(page)
 
-        if (page === pages.last()) {
+        // Preload next chapter once we're within the last 3 pages of the current chapter
-            Timber.d("Request preload next chapter because we're at the last page")
+        val inPreloadRange = pages.size - page.number < 3
+        if (inPreloadRange) {
+            Timber.d("Request preload next chapter because we're at page ${page.number} of ${pages.size}")
             adapter.nextTransition?.to?.let {
                 activity.requestPreloadChapter(it)
             }

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerViewerAdapter.kt

@@ -41,7 +41,7 @@ class PagerViewerAdapter(private val viewer: PagerViewer) : ViewPagerAdapter() {
             }
         }
 
-        // Skip transition page if  the chapter is loaded & current page is not a transition page
+        // Skip transition page if the chapter is loaded & current page is not a transition page
         if (forceTransition || chapters.prevChapter?.state !is ReaderChapter.State.Loaded) {
             newItems.add(ChapterTransition.Prev(chapters.currChapter, chapters.prevChapter))
         }

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/webtoon/WebtoonAdapter.kt

@@ -38,7 +38,7 @@ class WebtoonAdapter(val viewer: WebtoonViewer) : androidx.recyclerview.widget.R
             }
         }
 
-        // Skip transition page if  the chapter is loaded & current page is not a transition page
+        // Skip transition page if the chapter is loaded & current page is not a transition page
         if (forceTransition || chapters.prevChapter?.state !is ReaderChapter.State.Loaded) {
             newItems.add(ChapterTransition.Prev(chapters.currChapter, chapters.prevChapter))
         }

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/webtoon/WebtoonViewer.kt

@@ -142,9 +142,11 @@ class WebtoonViewer(val activity: ReaderActivity) : BaseViewer {
         Timber.d("onPageSelected: ${page.number}/${pages.size}")
         activity.onPageSelected(page)
 
-        if (page === pages.last()) {
+        // Preload next chapter once we're within the last 3 pages of the current chapter
-            Timber.d("Request preload next chapter because we're at the last page")
+        val inPreloadRange = pages.size - page.number < 3
-            val transition = adapter.items.getOrNull(position + 1) as? ChapterTransition.Next
+        if (inPreloadRange) {
+            Timber.d("Request preload next chapter because we're at page ${page.number} of ${pages.size}")
+            val transition = adapter.items.getOrNull(pages.size + 1) as? ChapterTransition.Next
             if (transition?.to != null) {
                 activity.requestPreloadChapter(transition.to)
             }