Add reaction picker (#5904)

webapp/components/rhs_thread.jsx

@@ -424,7 +424,10 @@ export default class RhsThread extends React.Component {
                     renderView={renderView}
                     onScroll={this.handleScroll}
                 >
-                    <div className='post-right__scroll'>
+                    <div
+                        ref='post-right__scroll'
+                        className='post-right__scroll'
+                    >
                         <DateSeparator
                             date={rootPostDay.toDateString()}
                         />