It’s do that after creating a single custom category, see the screenshot below. Ill try to remoe Paper2D once at home to see if the scrollbar disapear.
Thanks for the FClassIconFinder::RegisterIconSource/UnregisterIconSource and documentation tips

Best Regards
Edit: I tested on few computer for the scrollbar and it doesnt seems to happear on every computer even if the screen resolution is the same. Ill try to figure why its show up on some and not on others.