Do not scroll the columns area due to redirection (#4541)

Commit 9d1f8b9d6a scrolls the columns area
when the route changes since the user is likely to want to see the
rightmost column in such cases.

However, redirection is automatic and does not indicate users' intension.
Do not scroll the columns area due to one.

app/javascript/mastodon/features/ui/index.js

@@ -1,12 +1,12 @@
 import React from 'react';
 import classNames from 'classnames';
-import Redirect from 'react-router-dom/Redirect';
 import NotificationsContainer from './containers/notifications_container';
 import PropTypes from 'prop-types';
 import LoadingBarContainer from './containers/loading_bar_container';
 import TabsBar from './components/tabs_bar';
 import ModalContainer from './containers/modal_container';
 import { connect } from 'react-redux';
+import { Redirect, withRouter } from 'react-router-dom';
 import { isMobile } from '../../is_mobile';
 import { debounce } from 'lodash';
 import { uploadCompose } from '../../actions/compose';
@@ -48,6 +48,7 @@ const mapStateToProps = state => ({
 });
 
 @connect(mapStateToProps)
+@withRouter
 export default class UI extends React.PureComponent {
 
   static contextTypes = {
@@ -59,6 +60,7 @@ export default class UI extends React.PureComponent {
     children: PropTypes.node,
     systemFontUi: PropTypes.bool,
     isComposing: PropTypes.bool,
+    location: PropTypes.object,
   };
 
   state = {
@@ -168,6 +170,12 @@ export default class UI extends React.PureComponent {
     return true;
   }
 
+  componentDidUpdate (prevProps) {
+    if (![this.props.location.pathname, '/'].includes(prevProps.location.pathname)) {
+      this.columnsAreaNode.handleChildrenContentChange();
+    }
+  }
+
   componentWillUnmount () {
     window.removeEventListener('resize', this.handleResize);
     document.removeEventListener('dragenter', this.handleDragEnter);
@@ -181,6 +189,10 @@ export default class UI extends React.PureComponent {
     this.node = c;
   }
 
+  setColumnsAreaRef = (c) => {
+    this.columnsAreaNode = c.getWrappedInstance().getWrappedInstance();
+  }
+
   render () {
     const { width, draggingOver } = this.state;
     const { children } = this.props;
@@ -192,7 +204,7 @@ export default class UI extends React.PureComponent {
     return (
       <div className={className} ref={this.setRef}>
         <TabsBar />
-        <ColumnsAreaContainer singleColumn={isMobile(width)}>
+        <ColumnsAreaContainer ref={this.setColumnsAreaRef} singleColumn={isMobile(width)}>
           <WrappedSwitch>
             <Redirect from='/' to='/getting-started' exact />
             <WrappedRoute path='/getting-started' component={GettingStarted} content={children} />