Fix linked-comment auto scroll

## Ticket
6946: Linked-comment scroll position is inconsistent

## Issues
- If it is a deeply-nested reply, the positioning is incorrect.
- If there are pinned comments, the positioning is way off.
- If there is a delay in mounting, the positioning doesn't happen.
- When clicking on the comment's date to highlight it, the comment goes missing and the scroll position is weird.

## Changes
- Take into account that replies can be linked-comments.
- Perform a "one-time" search for the linked-comment after the initial fetch, if necessary. The expensive DOM search is only be executed minimally.

ui/component/comment/view.jsx

@@ -1,6 +1,7 @@
 // @flow
 import * as ICONS from 'constants/icons';
 import * as PAGES from 'constants/pages';
+import { COMMENT_HIGHLIGHTED } from 'constants/classnames';
 import { SORT_BY, COMMENT_PAGE_SIZE_REPLIES } from 'constants/comment';
 import { FF_MAX_CHARS_IN_COMMENT } from 'constants/form-field';
 import { SITE_NAME, SIMPLE_SITE, ENABLE_COMMENT_REACTIONS } from 'config';
@@ -220,7 +221,7 @@ function Comment(props: Props) {
     >
       <div
         className={classnames('comment__content', {
-          'comment--highlighted': linkedCommentId && linkedCommentId === commentId,
+          [COMMENT_HIGHLIGHTED]: linkedCommentId && linkedCommentId === commentId,
           'comment--slimed': slimedToDeath && !displayDeadComment,
         })}
       >

ui/component/commentsList/view.jsx

@@ -1,6 +1,7 @@
 // @flow
 import * as REACTION_TYPES from 'constants/reactions';
 import * as ICONS from 'constants/icons';
+import { COMMENT_HIGHLIGHTED } from 'constants/classnames';
 import { COMMENT_PAGE_SIZE_TOP_LEVEL, SORT_BY } from 'constants/comment';
 import React, { useEffect } from 'react';
 import classnames from 'classnames';
@@ -18,6 +19,9 @@ import { getChannelIdFromClaim } from 'util/claim';
 
 const DEBOUNCE_SCROLL_HANDLER_MS = 200;
 
+// "3" due to 2 separate fetches needed + 1 buffer just in case.
+const MAX_LINKED_COMMENT_SCROLL_ATTEMPTS = 3;
+
 function scaleToDevicePixelRatio(value) {
   const devicePixelRatio = window.devicePixelRatio || 1.0;
   if (devicePixelRatio < 1.0) {
@@ -75,11 +79,13 @@ function CommentList(props: Props) {
     settingsByChannelId,
   } = props;
 
-  const commentRef = React.useRef();
   const spinnerRef = React.useRef();
   const DEFAULT_SORT = ENABLE_COMMENT_REACTIONS ? SORT_BY.POPULARITY : SORT_BY.NEWEST;
   const [sort, setSort] = usePersistedState('comment-sort-by', DEFAULT_SORT);
   const [page, setPage] = React.useState(0);
+  const [lcScrollAttempts, setLcScrollAttempts] = React.useState(
+    linkedCommentId ? 0 : MAX_LINKED_COMMENT_SCROLL_ATTEMPTS
+  );
   const isMobile = useIsMobile();
   const [expandedComments, setExpandedComments] = React.useState(!isMobile);
   const totalFetchedComments = allCommentIds ? allCommentIds.length : 0;
@@ -196,11 +202,19 @@ function CommentList(props: Props) {
 
   // Scroll to linked-comment
   useEffect(() => {
-    if (readyToDisplayComments && linkedCommentId && commentRef && commentRef.current) {
-      commentRef.current.scrollIntoView({ block: 'start' });
-      window.scrollBy(0, -125);
+    if (lcScrollAttempts < MAX_LINKED_COMMENT_SCROLL_ATTEMPTS && readyToDisplayComments && !isFetchingComments) {
+      const elems = document.getElementsByClassName(COMMENT_HIGHLIGHTED);
+      if (elems.length > 0) {
+        setLcScrollAttempts(MAX_LINKED_COMMENT_SCROLL_ATTEMPTS);
+        const linkedComment = elems[0];
+        linkedComment.scrollIntoView({ block: 'start' });
+        window.scrollBy(0, -125);
+      } else {
+        setLcScrollAttempts(lcScrollAttempts + 1);
+      }
     }
-  }, [readyToDisplayComments, linkedCommentId]);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [readyToDisplayComments, isFetchingComments]); // We just want to respond to these, nothing else.
 
   // Infinite scroll
   useEffect(() => {
@@ -321,7 +335,6 @@ function CommentList(props: Props) {
               comments: expandedComments,
               'comments--contracted': !expandedComments,
             })}
-            ref={commentRef}
           >
             {readyToDisplayComments && pinnedComments && getCommentElems(pinnedComments)}
             {readyToDisplayComments && topLevelComments && getCommentElems(topLevelComments)}

ui/constants/classnames.js

@@ -0,0 +1 @@
+export const COMMENT_HIGHLIGHTED = 'comment--highlighted';