fix(gateway): resend thread member lists on resume and leave (#3372)

fluxer_gateway/test/guild_threads_tests.erl

@@ -1515,6 +1515,45 @@ thread_member_list_follows_member_updates_test() ->
         ?assertNot(maps:is_key(thread_list_dirty, Unlisted))
     end).
 
+thread_member_list_resubscribe_resends_snapshot_test() ->
+    with_guild(fun(State) ->
+        Subscribed = guild_thread_subscriptions:update(
+            <<"a">>, #{member_lists => [?T1]}, State
+        ),
+        [_] = received(a, thread_member_list_update, drain()),
+        Again = guild_thread_subscriptions:update(
+            <<"a">>, #{member_lists => [?T1]}, Subscribed
+        ),
+        [_] = received(a, thread_member_list_update, drain()),
+        _ = guild_thread_subscriptions:update(<<"a">>, #{threads => true}, Again),
+        ?assertEqual([], received(a, thread_member_list_update, drain()))
+    end).
+
+thread_member_list_drops_removed_guild_members_test() ->
+    with_guild(fun(State0) ->
+        Added = #{
+            <<"id">> => integer_to_binary(?T1),
+            <<"member_count">> => 2,
+            <<"added_members">> => [thread_member(?T1, ?V)]
+        },
+        State1 = dispatch(thread_members_update, Added, State0),
+        Subscribed = guild_thread_subscriptions:update(
+            <<"a">>, #{member_lists => [?T1]}, State1
+        ),
+        _ = drain(),
+        Removed = dispatch(
+            guild_member_remove,
+            #{<<"user">> => #{<<"id">> => integer_to_binary(?V)}},
+            Subscribed
+        ),
+        ?assertEqual(#{?T1 => true}, maps:get(thread_list_dirty, Removed)),
+        _ = guild_thread_subscriptions:handle_list_flush(Removed),
+        [#{<<"members">> := Members}] = received(a, thread_member_list_update, drain()),
+        ?assertEqual(
+            [integer_to_binary(?A)], [maps:get(<<"user_id">>, M) || M <- Members]
+        )
+    end).
+
 handoff_through_an_old_node_round_trips_test() ->
     with_guild(fun(State) ->
         Exported = guild_handoff:export_handoff_state(State),