Update, simplify and add smooth scrolling toggle on `SPC t v`

doc/DOCUMENTATION.org

@@ -919,6 +919,7 @@ and ~T~):
 | ~SPC t l~   | toggle truncate lines                                             |
 | ~SPC t L~   | toggle visual lines                                               |
 | ~SPC t n~   | toggle line numbers                                               |
+| ~SPC t v~   | toggle smooth scrolling                                           |
 
 | Key Binding | Description                                                  |
 |-------------+--------------------------------------------------------------|
@@ -1405,9 +1406,11 @@ On Windows, you may want to disable it. To disable the smooth scrolling set the
 =dotspacemacs-smooth-scrolling= variable in your =~/.spacemacs= to =nil=:
 
 #+BEGIN_SRC emacs-lisp
-(setq-default dotspacemacs-smooth-scrolling t)
+(setq-default dotspacemacs-smooth-scrolling nil)
 #+END_SRC
 
+You can also toggle smooth scrolling with ~SPC t v~.
+
 *** Vim motions with avy
 Spacemacs uses the =evil= integration of [[https://github.com/abo-abo/avy][avy]] which enables the
 invocation of =avy= during motions.

layers/+spacemacs/spacemacs-ui-visual/packages.el

@@ -233,32 +233,33 @@
     (spacemacs//neotree-key-bindings)))
 
 (defun spacemacs-ui-visual/init-smooth-scrolling ()
-  (defun spacemacs//unset-scroll-margin ()
-    "Set scroll-margin to zero."
-    (setq-local scroll-margin 0))
-
   (use-package smooth-scrolling
-    :if dotspacemacs-smooth-scrolling
-    :init (setq smooth-scroll-margin 5
-                scroll-conservatively 101
-                scroll-preserve-screen-position t
-                auto-window-vscroll nil)
-    :config
+    :init
     (progn
-      (setq scroll-margin 5)
+      (setq smooth-scroll-margin 5)
+      (spacemacs|add-toggle smooth-scrolling
+        :status smooth-scrolling-mode
+        :on (progn
+              (smooth-scrolling-mode)
+              (enable-smooth-scroll-for-function previous-line)
+              (enable-smooth-scroll-for-function next-line)
+              (enable-smooth-scroll-for-function isearch-repeat))
+        :off (progn
+               (smooth-scrolling-mode -1)
+               (disable-smooth-scroll-for-function previous-line)
+               (disable-smooth-scroll-for-function next-line)
+               (disable-smooth-scroll-for-function isearch-repeat))
+        :documentation "Smooth scrolling."
+        :evil-leader "tv")
+      (unless dotspacemacs-smooth-scrolling
+        (spacemacs/toggle-smooth-scrolling-off))
       ;; add hooks here only for emacs built-in packages that are not owned
       ;; by a layer.
+      (defun spacemacs//unset-scroll-margin ()
+        "Set scroll-margin to zero."
+        (setq-local scroll-margin 0))
       (spacemacs/add-to-hooks 'spacemacs//unset-scroll-margin
-                              '(messages-buffer-mode-hook))))
-
-  (unless dotspacemacs-smooth-scrolling
-    ;; deactivate smooth-scrolling advices
-    (ad-disable-advice 'previous-line 'after 'smooth-scroll-down)
-    (ad-activate 'previous-line)
-    (ad-disable-advice 'next-line 'after 'smooth-scroll-up)
-    (ad-activate 'next-line)
-    (ad-disable-advice 'isearch-repeat 'after 'isearch-smooth-scroll)
-    (ad-activate 'isearch-repeat)))
+                              '(messages-buffer-mode-hook)))))
 
 (defun spacemacs-ui-visual/init-spaceline ()
   (use-package spaceline-config