feat(smooth-scroll): add +interpolate flag

Adds optional scroll interpolation/animation in #8335 via the
good-scroll package. Command support is light right now, but I plan to
expand on it later, and I welcome PRs to do so in the meantime.

Ref: #8335

modules/ui/smooth-scroll/README.org

@@ -4,18 +4,21 @@
 #+since:    25.05.0
 
 * Description :unfold:
-This module activates OS-independent smooth scrolling in Emacs. This primarily
+This module enables multiple kinds of smooth scrolling in Emacs. Its primary
-benefits trackpad and scroll wheel users (not so much those that scroll via
+function is to make input scrolling (on trackpads and scroll wheels)
-their keyboards), by utilizing the [[https://github.com/jdtsmith/ultra-scroll][ultra-scroll]] package.
+pixel-smooth. With the =+interpolate= flag, it performs interpolated scrolling on
+a growing list of scroll commands that traverse larger distances, smoothly
+scrolling from point A to B on PgUp or PgDown (or [[kbd:][C-d]]/[[kbd:][C-u]] for Evil users).
 
 #+begin_quote
   This module requires Emacs 29.1 or newer.
 #+end_quote
 
 #+begin_quote
-  For optimal performance from this module, it's highly recommended you use
+  Scroll interpolation support is currently limited to the ~scroll-up~ and
-    Emacs with native-compilation. MacOS users may also have a better experience
+    ~scroll-down~ commands, and any command that calls them (like ~evil-scroll-down~
-    using the [[https://bitbucket.org/mituharu/emacs-mac][emacs-mac]] fork of Emacs, available via Homebrew.
+    and ~evil-scroll-up~), so you're likely to find many scroll commands are not
+    interpolated. We welcome PRs to expand its support.
 #+end_quote
 
 ** Maintainers
@@ -24,10 +27,14 @@ their keyboards), by utilizing the [[https://github.com/jdtsmith/ultra-scroll][u
 [[doom-contrib-maintainer:][Become a maintainer?]]
 
 ** Module flags
-/This module has no flags./
+- +interpolate ::
+  Enables scroll interpolation for some larger-step scrolling commands. E.g.
+  PgUp and PgDown (or C-d/C-u for Evil users) will now smoothly scroll to its
+  destination rather than jump to it.
 
 ** Packages
 - [[doom-package:ultra-scroll]]
+- [[doom-package:good-scroll]] if [[doom-module:+interpolate]]
 
 ** Hacks
 /No hacks documented for this module./
@@ -41,6 +48,12 @@ their keyboards), by utilizing the [[https://github.com/jdtsmith/ultra-scroll][u
 
 /This module has no external requirements./
 
+#+begin_quote
+  For optimal performance from this module, it's highly recommended you use
+    Emacs with native-compilation. MacOS users may also have a better experience
+    using the [[https://bitbucket.org/mituharu/emacs-mac][emacs-mac]] fork of Emacs, available via Homebrew.
+#+end_quote
+
 * Usage
 This module only needs to be activated to experience its benefits.
 

modules/ui/smooth-scroll/config.el

@@ -11,3 +11,23 @@
   (add-hook 'ultra-scroll-hide-functions #'hl-todo-mode)
   (add-hook 'ultra-scroll-hide-functions #'diff-hl-flydiff-mode)
   (add-hook 'ultra-scroll-hide-functions #'jit-lock-mode))
+
+
+(use-package good-scroll
+  :when (modulep! +interpolate)
+  :hook (doom-first-input . good-scroll-mode)
+  :config
+  (defun good-scroll--convert-line-to-step (line)
+    (cond ((integerp line) (* line (line-pixel-height)))
+          ((or (null line) (memq '- line))
+           (- (good-scroll--window-usable-height)
+              (* next-screen-context-lines (line-pixel-height))))
+          ((line-pixel-height))))
+
+  (defadvice! good-scroll--scroll-up (&optional arg)
+    :override #'scroll-up
+    (good-scroll-move (good-scroll--convert-line-to-step arg)))
+
+  (defadvice! good-scroll--scroll-down (&optional arg)
+    :override #'scroll-down
+    (good-scroll-move (- (good-scroll--convert-line-to-step arg)))))

modules/ui/smooth-scroll/packages.el

@@ -4,3 +4,6 @@
 (package! ultra-scroll
   :recipe (:host github :repo "jdtsmith/ultra-scroll")
   :pin "b72c507f6702db18d971a6b6bdc692e260f21159")
+
+(when (modulep! +interpolate)
+  (package! good-scroll :pin "a7ffd5c0e5935cebd545a0570f64949077f71ee3"))