When both horizontal and vertical ScrollBars are visible in Fluent theme, the "BottomHandle" RepeatButton cannot be pressed as it is overlapped by the horizontal ScrollBar on MouseOver. Default behavior will be changed and the scrollbars will be reordered in the ScrollViewer ControlTemplate so on MouseOver the current ScrollBar will have higher Canvas.ZIndex.
Hi, The fix for the issue will be available in next LIB version (2018.2.611). It will also be included in our next official version – R2 2018 SP1, scheduled for the end of June. Best regards, Ralitsa Kumanova