Scroll columns area to right when children property is changed (#4517)

The feature to pin column could hide the rightmost column, which is
specified with children property of ColumnsArea.

The user is likely to see the column when the property changed, so scroll
the area in such cases.

app/javascript/mastodon/features/ui/components/columns_area.js

@@ -12,6 +12,8 @@ import ColumnLoading from './column_loading';
 import BundleColumnError from './bundle_column_error';
 import { Compose, Notifications, HomeTimeline, CommunityTimeline, PublicTimeline, HashtagTimeline, FavouritedStatuses } from '../../ui/util/async-components';
 
+import { scrollRight } from '../../../scroll';
+
 const componentMap = {
   'COMPOSE': Compose,
   'HOME': HomeTimeline,
@@ -49,9 +51,13 @@ export default class ColumnsArea extends ImmutablePureComponent {
     this.setState({ shouldAnimate: true });
   }
 
-  componentDidUpdate() {
+  componentDidUpdate(prevProps) {
     this.lastIndex = getIndex(this.context.router.history.location.pathname);
     this.setState({ shouldAnimate: true });
+
+    if (this.props.children !== prevProps.children) {
+      scrollRight(this.node);
+    }
   }
 
   handleSwipe = (index) => {
@@ -74,6 +80,10 @@ export default class ColumnsArea extends ImmutablePureComponent {
     }
   }
 
+  setRef = (node) => {
+    this.node = node;
+  }
+
   renderView = (link, index) => {
     const columnIndex = getIndex(this.context.router.history.location.pathname);
     const title = this.props.intl.formatMessage({ id: link.props['data-preview-title-id'] });
@@ -114,7 +124,7 @@ export default class ColumnsArea extends ImmutablePureComponent {
     }
 
     return (
-      <div className='columns-area'>
+      <div className='columns-area' ref={this.setRef}>
         {columns.map(column => {
           const params = column.get('params', null) === null ? null : column.get('params').toJS();