module: add :ui smooth-scroll

Normally, this would be too small to warrant its own module, but smooth
scrolling is one of those things that can tip large swaths of
fence-sitting beginners into Emacs' camp. I can't help but imagine how
many people get their impression that Emacs is "laggy" or "slow" solely
from the way it scrolls.

Don't get me wrong, there *are* good reasons to call Emacs slow, but
startup and scrolling speed are not it, yet they are easily overblown
because it's the first thing you see. I might even consider enabling
this by default, but I'll make that decision later.

Fix: #8335

.github/CODEOWNERS

@@ -154,6 +154,7 @@ modules/ui/nav-flash                @doomemacs/ui-nav-flash
 modules/ui/neotree                  @doomemacs/ui-neotree
 modules/ui/ophints                  @doomemacs/ui-ophints
 modules/ui/popup                    @doomemacs/ui-popup
+modules/ui/smooth-scroll            @doomemacs/ui-smooth-scroll
 modules/ui/tabs                     @doomemacs/ui-tabs
 modules/ui/treemacs                 @doomemacs/ui-treemacs
 modules/ui/unicode                  @doomemacs/ui-unicode

modules/ui/smooth-scroll/README.org

@@ -0,0 +1,72 @@
+#+title:    :ui smooth-scroll
+#+subtitle: So smooth you won't believe it's not butter
+#+created:  April 1, 2025
+#+since:    25.05.0
+
+* Description :unfold:
+This module activates OS-independent smooth scrolling in Emacs. This primarily
+benefits trackpad and scroll wheel users (not so much those that scroll via
+their keyboards), by utilizing the [[https://github.com/jdtsmith/ultra-scroll][ultra-scroll]] package.
+
+#+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
+
+** Maintainers
+- [[doom-user:][@hlissner]]
+
+[[doom-contrib-maintainer:][Become a maintainer?]]
+
+** Module flags
+/This module has no flags./
+
+** Packages
+- [[doom-package:ultra-scroll]]
+
+** Hacks
+/No hacks documented for this module./
+
+** TODO Changelog
+# This section will be machine generated. Don't edit it by hand.
+/This module does not have a changelog yet./
+
+* Installation
+[[id:01cffea4-3329-45e2-a892-95a384ab2338][Enable this module in your ~doom!~ block.]]
+
+/This module has no external requirements./
+
+* Usage
+This module only needs to be activated to experience its benefits.
+
+* TODO Configuration
+#+begin_quote
+ 󱌣 This module has no configuration documentation yet. [[doom-contrib-module:][Write some?]]
+#+end_quote
+
+* Troubleshooting
+This module should work on any platform that provides pixel-level scrolling
+information from your input hardware, but if you run into issues, here are some
+things to try:
+
+- Use ~M-x ultra-scroll-check~ to diagnose common issues. Visit [[https://github.com/jdtsmith/ultra-scroll?tab=readme-ov-file#compatibility][ultra-scroll's
+  documentation]] for more on interpreting and acting on its results.
+- Read ultra-scroll's documentation on [[https://github.com/jdtsmith/ultra-scroll?tab=readme-ov-file#Speed][scrolling speed]]; it outlines common
+  factors that may impact it and how to mitigate them.
+- See Doom's "[[https://discourse.doomemacs.org/t/why-is-emacs-doom-slow/83/3][Why is Emacs/Doom slow?]]" write-up, where Doom's author documents
+  common factors that can slow Emacs down in general (not specifically to do
+  with scrolling) and what to do about them.
+
+If all else fails, let us know on [[https://discourse.doomemacs.org][Discourse]] or [[https://doomemacs.org/discord][Discord]]. Only file an issue if
+you have a good idea what's causing the problem or have an explicit error to
+report (performance issues, in general, tend to be the result of *many* factors
+and are the greatest source of false positives on our issue tracker).
+
+* Frequently asked questions
+/This module has no FAQs yet./ [[doom-suggest-faq:][Ask one?]]
+
+* TODO Appendix
+#+begin_quote
+ 󱌣 This module has no appendix yet. [[doom-contrib-module:][Write one?]]
+#+end_quote

modules/ui/smooth-scroll/config.el

@@ -0,0 +1,8 @@
+;;; ui/smooth-scroll/config.el -*- lexical-binding: t; -*-
+
+(use-package! ultra-scroll
+  :hook (doom-first-input . ultra-scroll-mode)
+  :hook (doom-first-file . ultra-scroll-mode)
+  :init
+  (setq scroll-conservatively 101
+        scroll-margin 0))

modules/ui/smooth-scroll/packages.el

@@ -0,0 +1,6 @@
+;; -*- no-byte-compile: t; -*-
+;;; ui/smooth-scroll/packages.el
+
+(package! ultra-scroll
+  :recipe (:host github :repo "jdtsmith/ultra-scroll")
+  :pin "b72c507f6702db18d971a6b6bdc692e260f21159")

static/init.example.el

@@ -43,6 +43,7 @@
        ;;neotree           ; a project drawer, like NERDTree for vim
        ophints           ; highlight the region an operation acts on
        (popup +defaults)   ; tame sudden yet inevitable temporary windows
+       ;;smooth-scroll     ; So smooth you won't believe it's not butter
        ;;tabs              ; a tab bar for Emacs
        ;;treemacs          ; a project drawer, like neotree but cooler
        ;;unicode           ; extended unicode support for various languages