Do not switch chapters if the current chapter is the first or last chapter in the chapter group.

lib/pages/reader/gesture.dart

@@ -131,11 +131,11 @@ class _ReaderGestureDetectorState extends AutomaticGlobalState<_ReaderGestureDet
     }
     if (context.reader.mode.key.startsWith('gallery')) {
       if (forward) {
-        if (!context.reader.toNextPage()) {
+        if (!context.reader.toNextPage() && !context.reader.isLastChapterOfGroup) {
           context.reader.toNextChapter();
         }
       } else {
-        if (!context.reader.toPrevPage()) {
+        if (!context.reader.toPrevPage() && !context.reader.isFirstChapterOfGroup) {
           context.reader.toPrevChapter();
         }
       }

lib/pages/reader/images.dart

@@ -206,11 +206,11 @@ class _GalleryModeState extends State<_GalleryMode>
       ),
       onPageChanged: (i) {
         if (i == 0) {
-          if (!reader.toPrevChapter()) {
+          if (reader.isFirstChapterOfGroup || !reader.toPrevChapter()) {
             reader.toPage(1);
           }
         } else if (i == totalPages + 1) {
-          if (!reader.toNextChapter()) {
+          if (reader.isLastChapterOfGroup || !reader.toNextChapter()) {
             reader.toPage(totalPages);
           }
         } else {
@@ -575,15 +575,14 @@ class _ContinuousModeState extends State<_ContinuousMode>
         }
 
         if (notification is ScrollUpdateNotification) {
-          var length = reader.maxChapter;
           if (!scrollController.hasClients) return false;
           if (scrollController.position.pixels <=
               scrollController.position.minScrollExtent &&
-              reader.chapter != 1) {
+              !reader.isFirstChapterOfGroup) {
             context.readerScaffold.setFloatingButton(-1);
           } else if (scrollController.position.pixels >=
               scrollController.position.maxScrollExtent &&
-              reader.chapter < length) {
+              !reader.isLastChapterOfGroup) {
             context.readerScaffold.setFloatingButton(1);
           } else {
             context.readerScaffold.setFloatingButton(0);

lib/pages/reader/reader.dart

@@ -265,6 +265,40 @@ class _ReaderState extends State<Reader>
       });
     }
   }
+
+  bool get isFirstChapterOfGroup {
+    if (widget.chapters?.isGrouped ?? false) {
+      int c = chapter - 1;
+      int g = 1;
+      while (c > 0) {
+        c -= widget.chapters!.getGroupByIndex(g - 1).length;
+        g++;
+      }
+      if (c == 0) {
+        return true;
+      } else {
+        return false;
+      }
+    }
+    return chapter == 1;
+  }
+
+  bool get isLastChapterOfGroup {
+    if (widget.chapters?.isGrouped ?? false) {
+      int c = chapter;
+      int g = 1;
+      while (c > 0) {
+        c -= widget.chapters!.getGroupByIndex(g - 1).length;
+        g++;
+      }
+      if (c == 0) {
+        return true;
+      } else {
+        return false;
+      }
+    }
+    return chapter == maxChapter;
+  }
 }
 
 abstract mixin class _ImagePerPageHandler {