[defaults] Show scroll bar when using the mouse wheel

Thanks to duianto

layers/+spacemacs/spacemacs-defaults/funcs.el

@@ -1433,6 +1433,23 @@ using a visual block/rectangle selection."
   (interactive)
   (spacemacs/sort-lines-by-column -1))
 
+;; Show scroll bar when using the mouse wheel
+(defun spacemacs//scroll-bar-hide ()
+  " Hide the scroll bar."
+  (scroll-bar-mode -1))
+(defun spacemacs//scroll-bar-show-delayed-hide (&rest _ignore)
+  "Show the scroll bar for a couple of seconds, before hiding it.
+
+This can be used to temporarily show the scroll bar when mouse wheel scrolling.
+(advice-add 'mwheel-scroll :after #'spacemacs//scroll-bar-show-delayed-hide)
+
+The advice can be removed with:
+(advice-remove 'mwheel-scroll #'spacemacs//scroll-bar-show-delayed-hide)"
+  (scroll-bar-mode 1)
+  (run-with-idle-timer 3 nil #'spacemacs//scroll-bar-hide))
+(when (fboundp 'scroll-bar-mode)
+  (advice-add 'mwheel-scroll :after #'spacemacs//scroll-bar-show-delayed-hide))
+
 ;; BEGIN linum mouse helpers
 
 (defvar spacemacs-linum-mdown-line nil