It is possible to add so many variables or array variable elements that the bottom of the list is not visible, and there are no scroll bars.
1. Make a new User Defined Struct
2. Add many variables to the struct with the "New Variable" button. Making the Struct Editor small makes this easier to test.
Expected: Being able to scroll to see the entire variable list.
Observed: The bottom of the variable list is cut off.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-38599 in the post.