As you may already know, UMG provides a built-in Scrollbox widget for easily displaying scrollable content. This widget handles all of the hard bits and gives you the ability to, mostly; customize its look to your liking. However, the scrollbar behavior is set in stone from UMGs side of things. Meaning you cannot have a scrollbar that has a constant size and does not change opacity on rollover.
NOTE: All Images shown here are WORK-IN-PROGRESS and do not represent the final product