Cancel outdated pending compose suggestions (#6838)

app/javascript/mastodon/actions/compose.js

 import api from '../api';
+import { CancelToken } from 'axios';
 import { throttle } from 'lodash';
 import { search as emojiSearch } from '../features/emoji/emoji_mart_search_light';
 import { tagHistory } from '../settings';
@@ -11,6 +12,8 @@ import {
   refreshPublicTimeline,
 } from './timelines';
 
+let cancelFetchComposeSuggestionsAccounts;
+
 export const COMPOSE_CHANGE          = 'COMPOSE_CHANGE';
 export const COMPOSE_SUBMIT_REQUEST  = 'COMPOSE_SUBMIT_REQUEST';
 export const COMPOSE_SUBMIT_SUCCESS  = 'COMPOSE_SUBMIT_SUCCESS';
@@ -257,13 +260,22 @@ export function undoUploadCompose(media_id) {
 };
 
 export function clearComposeSuggestions() {
+  if (cancelFetchComposeSuggestionsAccounts) {
+    cancelFetchComposeSuggestionsAccounts();
+  }
   return {
     type: COMPOSE_SUGGESTIONS_CLEAR,
   };
 };
 
 const fetchComposeSuggestionsAccounts = throttle((dispatch, getState, token) => {
+  if (cancelFetchComposeSuggestionsAccounts) {
+    cancelFetchComposeSuggestionsAccounts();
+  }
   api(getState).get('/api/v1/accounts/search', {
+    cancelToken: new CancelToken(cancel => {
+      cancelFetchComposeSuggestionsAccounts = cancel;
+    }),
     params: {
       q: token.slice(1),
       resolve: false,