(I am reposting this because I was logged in under the wrong account when I did it the first time ...)
I had been avoiding using the "Web Map" widget (ArcGIS Hub site builder) because it didn't have an option to add a legend. I just discovered that they added a legend option, which is terrific! However, now when you click on a feature in the map, the feature's pop-up box is half hidden beneath the legend. Also, if you click on something with a pop-up, and then click away from it, the legend disappears entirely. It then comes back if you click on a feature with a pop-up. I believe this is a bug.