scroll last comment into view

app/src/main/java/org/schabi/newpipe/MainActivity.java

@@ -563,10 +563,7 @@ public class MainActivity extends AppCompatActivity {
                 // to show the top level comments again.
                 final FragmentManager.BackStackEntry bse = fm.getBackStackEntryAt(
                         fm.getBackStackEntryCount() - 2); // current fragment is at the top
-                if (!CommentRepliesFragment.TAG.equals(bse.getName())) {
+                openDetailFragmentFromCommentReplies(fm, bse);
-                    BottomSheetBehavior.from(mainBinding.fragmentPlayerHolder)
-                            .setState(BottomSheetBehavior.STATE_EXPANDED);
-                }
             }
 
         } else {
@@ -652,10 +649,7 @@ public class MainActivity extends AppCompatActivity {
             fm.popBackStackImmediate();
             final FragmentManager.BackStackEntry bse = fm.getBackStackEntryAt(
                     fm.getBackStackEntryCount() - 1);
-            if (!CommentRepliesFragment.TAG.equals(bse.getName())) {
+            openDetailFragmentFromCommentReplies(fm, bse);
-                BottomSheetBehavior.from(mainBinding.fragmentPlayerHolder)
-                        .setState(BottomSheetBehavior.STATE_EXPANDED);
-            }
         } else if (!NavigationHelper.tryGotoSearchFragment(fm)) {
             // If search fragment wasn't found in the backstack go to the main fragment
             NavigationHelper.gotoMainFragment(fm);
@@ -854,6 +848,42 @@ public class MainActivity extends AppCompatActivity {
         }
     }
 
+    private void openDetailFragmentFromCommentReplies(
+            @NonNull final FragmentManager fm,
+            @NonNull final FragmentManager.BackStackEntry bse) {
+        if (!CommentRepliesFragment.TAG.equals(bse.getName())) {
+            final CommentRepliesFragment commentRepliesFragment =
+                    (CommentRepliesFragment) fm.findFragmentByTag(
+                            CommentRepliesFragment.TAG);
+            final BottomSheetBehavior bsb = BottomSheetBehavior
+                    .from(mainBinding.fragmentPlayerHolder);
+            bsb.addBottomSheetCallback(new BottomSheetBehavior.BottomSheetCallback() {
+                @Override
+                public void onStateChanged(@NonNull final View bottomSheet,
+                                           final int newState) {
+                    if (newState == BottomSheetBehavior.STATE_EXPANDED) {
+                        final Fragment detailFragment = fm.findFragmentById(
+                                R.id.fragment_player_holder);
+                        if (detailFragment instanceof VideoDetailFragment
+                                && commentRepliesFragment != null) {
+                            // should always be the case
+                            ((VideoDetailFragment) detailFragment).scrollToComment(
+                                    commentRepliesFragment.getCommentsInfoItem());
+                        }
+                        bsb.removeBottomSheetCallback(this);
+                    }
+                }
+
+                @Override
+                public void onSlide(@NonNull final View bottomSheet,
+                                    final float slideOffset) {
+                    // not needed, listener is removed once the sheet is expanded
+                }
+            });
+            bsb.setState(BottomSheetBehavior.STATE_EXPANDED);
+        }
+    }
+
     private boolean bottomSheetHiddenOrCollapsed() {
         final BottomSheetBehavior<FrameLayout> bottomSheetBehavior =
                 BottomSheetBehavior.from(mainBinding.fragmentPlayerHolder);

app/src/main/java/org/schabi/newpipe/fragments/detail/VideoDetailFragment.java

@@ -74,6 +74,7 @@ import org.schabi.newpipe.error.UserAction;
 import org.schabi.newpipe.extractor.Image;
 import org.schabi.newpipe.extractor.InfoItem;
 import org.schabi.newpipe.extractor.NewPipe;
+import org.schabi.newpipe.extractor.comments.CommentsInfoItem;
 import org.schabi.newpipe.extractor.exceptions.ContentNotSupportedException;
 import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.stream.AudioStream;
@@ -1012,6 +1013,14 @@ public final class VideoDetailFragment
         updateTabLayoutVisibility();
     }
 
+    public void scrollToComment(final CommentsInfoItem comment) {
+        final Fragment fragment = pageAdapter.getItem(
+                pageAdapter.getItemPositionByTitle(COMMENTS_TAB_TAG));
+        if (fragment instanceof CommentsFragment) {
+            ((CommentsFragment) fragment).scrollToComment(comment);
+        }
+    }
+
     /*//////////////////////////////////////////////////////////////////////////
     // Play Utils
     //////////////////////////////////////////////////////////////////////////*/

app/src/main/java/org/schabi/newpipe/fragments/list/comments/CommentRepliesFragment.java

@@ -159,4 +159,10 @@ public final class CommentRepliesFragment
         return ItemViewMode.LIST;
     }
 
+    /**
+     * @return the comment to which the replies are shown
+     */
+    public CommentsInfoItem getCommentsInfoItem() {
+        return commentsInfoItem;
+    }
 }

app/src/main/java/org/schabi/newpipe/fragments/list/comments/CommentsFragment.java

@@ -110,4 +110,8 @@ public class CommentsFragment extends BaseListInfoFragment<CommentsInfoItem, Com
     protected ItemViewMode getItemViewMode() {
         return ItemViewMode.LIST;
     }
+
+    public void scrollToComment(final CommentsInfoItem comment) {
+        itemsList.scrollToPosition(infoListAdapter.getItemsList().indexOf(comment));
+    }
 }

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -504,7 +504,8 @@ public final class NavigationHelper {
     public static void openCommentRepliesFragment(@NonNull final FragmentActivity activity,
                                                   final CommentsInfoItem commentsInfoItem) {
         defaultTransaction(activity.getSupportFragmentManager())
-                .replace(R.id.fragment_holder, new CommentRepliesFragment(commentsInfoItem))
+                .replace(R.id.fragment_holder, new CommentRepliesFragment(commentsInfoItem),
+                        CommentRepliesFragment.TAG)
                 .addToBackStack(CommentRepliesFragment.TAG)
                 .commit();
     }