Fix direct timeline pagination in the WebUI (#10126)

The `hasMore` property of timelines in redux store was set whenever an API
request returned only one page of results, *even* if the query only requested
newer conversations (using `since_id`), causing `hasMore` to be incorrectly set to
false whenever fetching new toots in the direct timeline, which happens each time
the direct message column is opened.

(Basically #9516 for direct messages)

app/javascript/mastodon/actions/conversations.js

@@ -41,13 +41,15 @@ export const expandConversations = ({ maxId } = {}) => (dispatch, getState) => {
     params.since_id = getState().getIn(['conversations', 'items', 0, 'last_status']);
   }
 
+  const isLoadingRecent = !!params.since_id;
+
   api(getState).get('/api/v1/conversations', { params })
     .then(response => {
       const next = getLinks(response).refs.find(link => link.rel === 'next');
 
       dispatch(importFetchedAccounts(response.data.reduce((aggr, item) => aggr.concat(item.accounts), [])));
       dispatch(importFetchedStatuses(response.data.map(item => item.last_status).filter(x => !!x)));
-      dispatch(expandConversationsSuccess(response.data, next ? next.uri : null));
+      dispatch(expandConversationsSuccess(response.data, next ? next.uri : null, isLoadingRecent));
     })
     .catch(err => dispatch(expandConversationsFail(err)));
 };
@@ -56,10 +58,11 @@ export const expandConversationsRequest = () => ({
   type: CONVERSATIONS_FETCH_REQUEST,
 });
 
-export const expandConversationsSuccess = (conversations, next) => ({
+export const expandConversationsSuccess = (conversations, next, isLoadingRecent) => ({
   type: CONVERSATIONS_FETCH_SUCCESS,
   conversations,
   next,
+  isLoadingRecent,
 });
 
 export const expandConversationsFail = error => ({

app/javascript/mastodon/reducers/conversations.js

@@ -35,7 +35,7 @@ const updateConversation = (state, item) => state.update('items', list => {
   }
 });
 
-const expandNormalizedConversations = (state, conversations, next) => {
+const expandNormalizedConversations = (state, conversations, next, isLoadingRecent) => {
   let items = ImmutableList(conversations.map(conversationToMap));
 
   return state.withMutations(mutable => {
@@ -66,7 +66,7 @@ const expandNormalizedConversations = (state, conversations, next) => {
       });
     }
 
-    if (!next) {
+    if (!next && !isLoadingRecent) {
       mutable.set('hasMore', false);
     }
 
@@ -81,7 +81,7 @@ export default function conversations(state = initialState, action) {
   case CONVERSATIONS_FETCH_FAIL:
     return state.set('isLoading', false);
   case CONVERSATIONS_FETCH_SUCCESS:
-    return expandNormalizedConversations(state, action.conversations, action.next);
+    return expandNormalizedConversations(state, action.conversations, action.next, action.isLoadingRecent);
   case CONVERSATIONS_UPDATE:
     return updateConversation(state, action.conversation);
   case CONVERSATIONS_MOUNT: