persist last-intersected status update and restore when ScrollableList is restored

e.g. when navigating from home-timeline to a status conversational  thread and <Back again

app/javascript/mastodon/actions/timelines.js

@@ -17,6 +17,14 @@ export const TIMELINE_LOAD_PENDING = 'TIMELINE_LOAD_PENDING';
 export const TIMELINE_DISCONNECT   = 'TIMELINE_DISCONNECT';
 export const TIMELINE_CONNECT      = 'TIMELINE_CONNECT';
 
+export const CURRENTLY_VIEWING = 'CURRENTLY_VIEWING';
+
+export const updateCurrentlyViewing = (timeline, id) => ({
+  type: CURRENTLY_VIEWING,
+  timeline,
+  id,
+});
+
 export const loadPending = timeline => ({
   type: TIMELINE_LOAD_PENDING,
   timeline,

app/javascript/mastodon/components/intersection_observer_article.js

@@ -20,6 +20,8 @@ export default class IntersectionObserverArticle extends React.Component {
     cachedHeight: PropTypes.number,
     onHeightChange: PropTypes.func,
     children: PropTypes.node,
+    currentlyViewing: PropTypes.number,
+    updateCurrentlyViewing: PropTypes.func,
   };
 
   state = {
@@ -48,6 +50,8 @@ export default class IntersectionObserverArticle extends React.Component {
     );
 
     this.componentMounted = true;
+
+    if(id === this.props.currentlyViewing) this.node.scrollIntoView();
   }
 
   componentWillUnmount () {
@@ -60,6 +64,8 @@ export default class IntersectionObserverArticle extends React.Component {
   handleIntersection = (entry) => {
     this.entry = entry;
 
+    if(entry.intersectionRatio > 0.75 && this.props.updateCurrentlyViewing) this.props.updateCurrentlyViewing(this.id);
+
     scheduleIdleTask(this.calculateHeight);
     this.setState(this.updateStateAfterIntersection);
   }

app/javascript/mastodon/components/scrollable_list.js

@@ -36,6 +36,8 @@ export default class ScrollableList extends PureComponent {
     emptyMessage: PropTypes.node,
     children: PropTypes.node,
     bindToDocument: PropTypes.bool,
+    currentlyViewing: PropTypes.number,
+    updateCurrentlyViewing: PropTypes.func,
   };
 
   static defaultProps = {
@@ -309,6 +311,8 @@ export default class ScrollableList extends PureComponent {
                 listLength={childrenCount}
                 intersectionObserverWrapper={this.intersectionObserverWrapper}
                 saveHeightKey={trackScroll ? `${this.context.router.route.location.key}:${scrollKey}` : null}
+                currentlyViewing={this.props.currentlyViewing}
+                updateCurrentlyViewing={this.props.updateCurrentlyViewing}
               >
                 {React.cloneElement(child, {
                   getScrollPosition: this.getScrollPosition,

app/javascript/mastodon/components/status_list.js

@@ -26,6 +26,8 @@ export default class StatusList extends ImmutablePureComponent {
     emptyMessage: PropTypes.node,
     alwaysPrepend: PropTypes.bool,
     timelineId: PropTypes.string,
+    currentlyViewing: PropTypes.number,
+    updateCurrentlyViewing: PropTypes.func,
   };
 
   static defaultProps = {

app/javascript/mastodon/features/ui/containers/status_list_container.js

@@ -1,6 +1,6 @@
 import { connect } from 'react-redux';
 import StatusList from '../../../components/status_list';
-import { scrollTopTimeline, loadPending } from '../../../actions/timelines';
+import { scrollTopTimeline, loadPending, updateCurrentlyViewing } from '../../../actions/timelines';
 import { Map as ImmutableMap, List as ImmutableList } from 'immutable';
 import { createSelector } from 'reselect';
 import { debounce } from 'lodash';
@@ -39,6 +39,7 @@ const makeMapStateToProps = () => {
     isPartial: state.getIn(['timelines', timelineId, 'isPartial'], false),
     hasMore:   state.getIn(['timelines', timelineId, 'hasMore']),
     numPending: getPendingStatusIds(state, { type: timelineId }).size,
+    currentlyViewing: state.getIn(['timelines', timelineId, 'currentlyViewing'], -1),
   });
 
   return mapStateToProps;
@@ -56,6 +57,7 @@ const mapDispatchToProps = (dispatch, { timelineId }) => ({
 
   onLoadPending: () => dispatch(loadPending(timelineId)),
 
+  updateCurrentlyViewing: id => dispatch(updateCurrentlyViewing(timelineId, id)),
 });
 
 export default connect(makeMapStateToProps, mapDispatchToProps)(StatusList);

app/javascript/mastodon/reducers/timelines.js

@@ -9,6 +9,7 @@ import {
   TIMELINE_CONNECT,
   TIMELINE_DISCONNECT,
   TIMELINE_LOAD_PENDING,
+  CURRENTLY_VIEWING,
 } from '../actions/timelines';
 import {
   ACCOUNT_BLOCK_SUCCESS,
@@ -28,6 +29,7 @@ const initialTimeline = ImmutableMap({
   hasMore: true,
   pendingItems: ImmutableList(),
   items: ImmutableList(),
+  currentlyViewing: -1,
 });
 
 const expandNormalizedTimeline = (state, timeline, statuses, next, isPartial, isLoadingRecent, usePendingItems) => {
@@ -168,6 +170,8 @@ export default function timelines(state = initialState, action) {
       initialTimeline,
       map => map.set('online', false).update(action.usePendingItems ? 'pendingItems' : 'items', items => items.first() ? items.unshift(null) : items)
     );
+  case CURRENTLY_VIEWING:
+    return state.update(action.timeline, initialTimeline, map => map.set('currentlyViewing', action.id));
   default:
     return state;
   }