This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
A user has reported an issue about scroll bars having a minimum size in UMG and Slate code, causing them to not scale correctly when the scroll box is at a smaller size. This affects both horizontal and vertical orientations.
This also occurred in 4.14
Head over to the existing Questions & Answers thread and let us know what's up.