I read that Rabbi Moshe Sternbuch said the following (while teaching the laws of this year’s Tisha Be’av, which fell out on Shabbos and was postponed to Sunday):
Our custom is to sing less Zemiros this Shabbos. But not so fewer than usual this would be noticeable, since that would be “public mourning”, forbidden on Shabbos.
The logic here seems very strange. And by “our custom” does he mean himself? Either nothing whatsoever is gained by less singing or he violates the prohibition of mourning, especially by telling us what he is doing.
Also, he must sing quite a few to be able to tell what “somewhat fewer” is…