In this Stackoverflow question, messages seem to be logically grouped inside some kind of elements named "firstgroup" and "secondgroup".
Are these elements standard UML sequence diagram elements? If yes, what are they called?
They look like combined fragments. However, such fragments must have an operator. Frequent operators are opt, alt or par. firstgroup is not among them. So, I think the shown diagram is not legal UML.
Some tools might allow to add arbitrary graphical shapes to the diagram, but they have no formal meaning. I would recommend not to use shapes that can be confused with regular notation, as is the case here.