Watching Roberta for another time, I noticed this dance sequence was done with no cuts and no close-ups. Hard to do and takes a lot of practice for a 3 minute number. Fred used to insist on no close-ups so that the viewer would see all of the steps.
He also had the studio install an oak floor so that the tap steps could be recorded at the same time as the filming. Ordinarily the taps sounds are added later.