This seems to be a regression introduced with the new page formatting.
When there are a large amount of students in a course the titles for each of the activities stays sticky to the top as you scroll down.
Also, when you have a lot of activities, as you scroll across the user details would stay sticky to the left.
Both of these features no longer work.
Additionally when loading the gradebook with a lot of users, the "Overall average" column is duplicated and stuck part way down the page (see screenshot).