ICC18 version of reaper on MacOs 10.12 has much quicker scrolling of menus (great!) but in the menu editor there is a bug. Switch from toolbar editing to menu and scrolling down makes the last edited toolbar visible under.
And in case it matters, I'm using a Magic Mouse (v1)
Tested with:
5.93RC1 ICC18 (bug)
5.80 release (no bug, slow scroll)
5.80 ICC18 (bug)
5.90 release (no bug slow scroll)
Comparison of scrolling speed 5.90 on left, 5.80ICC18 on right
https://imgur.com/RLYEsks