diff --git a/web/selectors/thread-selectors.js b/web/selectors/thread-selectors.js
--- a/web/selectors/thread-selectors.js
+++ b/web/selectors/thread-selectors.js
@@ -3,6 +3,7 @@
 import invariant from 'invariant';
 import * as React from 'react';
 import { useDispatch } from 'react-redux';
+import { createSelector } from 'reselect';
 
 import { ENSCacheContext } from 'lib/components/ens-cache-provider.react';
 import { createPendingSidebar } from 'lib/shared/thread-utils';
@@ -14,6 +15,7 @@
 
 import { getDefaultTextMessageRules } from '../markdown/rules.react';
 import { updateNavInfoActionType } from '../redux/action-types';
+import type { AppState, CalendarCommunityFilter } from '../redux/redux-setup';
 import { useSelector } from '../redux/redux-utils';
 
 function useOnClickThread(
@@ -107,9 +109,22 @@
   );
 }
 
+const filteredCommunityThreadIDsSelector: (
+  state: AppState,
+) => ?$ReadOnlySet<string> = createSelector(
+  (state: AppState) => state.communityFilter,
+  (communityFilter: ?CalendarCommunityFilter): ?Set<string> => {
+    if (!communityFilter || communityFilter.threadIDs.length === 0) {
+      return null;
+    }
+    return new Set(communityFilter.threadIDs);
+  },
+);
+
 export {
   useOnClickThread,
   useThreadIsActive,
   useOnClickPendingSidebar,
   useOnClickNewThread,
+  filteredCommunityThreadIDsSelector,
 };