Fix scroll to top in single column UI (#11463)

app/javascript/mastodon/components/column_back_button.js

@@ -2,6 +2,7 @@ import React from 'react';
 import { FormattedMessage } from 'react-intl';
 import PropTypes from 'prop-types';
 import Icon from 'mastodon/components/icon';
+import { createPortal } from 'react-dom';
 
 export default class ColumnBackButton extends React.PureComponent {
 
@@ -9,6 +10,10 @@ export default class ColumnBackButton extends React.PureComponent {
     router: PropTypes.object,
   };
 
+  static propTypes = {
+    multiColumn: PropTypes.bool,
+  };
+
   handleClick = () => {
     if (window.history && window.history.length === 1) {
       this.context.router.history.push('/');
@@ -18,12 +23,20 @@ export default class ColumnBackButton extends React.PureComponent {
   }
 
   render () {
-    return (
+    const { multiColumn } = this.props;
+
+    const component = (
       <button onClick={this.handleClick} className='column-back-button'>
         <Icon id='chevron-left' className='column-back-button__icon' fixedWidth />
         <FormattedMessage id='column_back_button.label' defaultMessage='Back' />
       </button>
     );
+
+    if (multiColumn) {
+      return component;
+    } else {
+      return createPortal(component, document.getElementById('tabs-bar__portal'));
+    }
   }
 
 }