PLT-5077: Fix floating date on mobile view. (#5061)

webapp/components/post_view/components/post.jsx

@@ -250,7 +250,11 @@ export default class Post extends React.Component {
         }
 
         return (
-            <div>
+            <div
+                ref={(div) => {
+                    this.domNode = div;
+                }}
+            >
                 <div
                     id={'post_' + post.id}
                     className={'post ' + sameUserClass + ' ' + compactClass + ' ' + rootUser + ' ' + postType + ' ' + currentUserCss + ' ' + shouldHighlightClass + ' ' + systemMessageClass + ' ' + hideControls + ' ' + dropdownOpenedClass}

webapp/components/post_view/components/post_list.jsx

@@ -159,7 +159,7 @@ export default class PostList extends React.Component {
                 const id = this.props.postList.order[i];
                 const element = this.refs[id];
 
-                if (!element || element.offsetTop + element.clientHeight <= this.refs.postlist.scrollTop) {
+                if (!element || !element.domNode || element.domNode.offsetTop + element.domNode.clientHeight <= this.refs.postlist.scrollTop) {
                     // this post is off the top of the screen so the last one is at the top of the screen
                     let topPostId;