diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -1768,30 +1768,12 @@
   return chatMentionCandidatesObj;
 }
 
+const emptyMentionCandidates = {};
 function useThreadChatMentionCandidates(
+  // eslint-disable-next-line no-unused-vars
   threadInfo: ThreadInfo,
 ): ChatMentionCandidates {
-  const { chatMentionCandidatesObj, communityThreadIDForGenesisThreads } =
-    useChatMentionCandidatesObjAndUtils();
-  return React.useMemo(() => {
-    let communityID,
-      result = {};
-    if (threadInfo.community === genesis.id) {
-      communityID = communityThreadIDForGenesisThreads[threadInfo.id];
-    } else {
-      communityID = threadInfo.community ?? threadInfo.id;
-    }
-    if (chatMentionCandidatesObj[communityID]) {
-      result = { ...chatMentionCandidatesObj[communityID] };
-    }
-    delete result[threadInfo.id];
-    return result;
-  }, [
-    chatMentionCandidatesObj,
-    communityThreadIDForGenesisThreads,
-    threadInfo.community,
-    threadInfo.id,
-  ]);
+  return emptyMentionCandidates;
 }
 
 function useUserProfileThreadInfo(userInfo: ?UserInfo): ?UserProfileThreadInfo {
@@ -1852,6 +1834,7 @@
   ]);
 }
 
+// eslint-disable-next-line no-unused-vars
 function useChatMentionSearchIndex(): {
   +[id: string]: SentencePrefixSearchIndex,
 } {
@@ -1880,20 +1863,12 @@
   }, [chatMentionCandidatesObj]);
 }
 
+const emptySearchIndex = new SentencePrefixSearchIndex();
 function useThreadChatMentionSearchIndex(
+  // eslint-disable-next-line no-unused-vars
   threadInfo: ThreadInfo,
 ): SentencePrefixSearchIndex {
-  const chatMentionCandidatesSearchIndex = useChatMentionSearchIndex();
-  const { communityThreadIDForGenesisThreads } =
-    useChatMentionCandidatesObjAndUtils();
-  if (threadInfo.community === genesis.id) {
-    return chatMentionCandidatesSearchIndex[
-      communityThreadIDForGenesisThreads[threadInfo.id]
-    ];
-  }
-  return chatMentionCandidatesSearchIndex[
-    threadInfo.community ?? threadInfo.id
-  ];
+  return emptySearchIndex;
 }
 
 export {