I searched for scroll in the Spacemacs source and found that this is a recently added feature:
(spacemacs|defc dotspacemacs-scroll-bar-while-scrolling t
"Show the scroll bar while scrolling. The auto hide time can be configured by
setting this variable to a number."
'(choice boolean number)
You can disable it in ...
I did not find a way to integrate it into the Spacemacs intended use of packages.
Adding this snippet to my dotspacemacs/user-config:
:init (doom-modeline-mode 1)
;; Enable flashing mode-line on errors
;; Corrects (and improves) org-mode's native fontification....