feat: add scrollbar for virtualized lists

crates/theme/src/resolve.rs

@@ -238,7 +238,10 @@ pub(crate) fn resolve(
         radius,
         elevations: Elevations::for_platform(system.source, variant),
         motion: Motion::default(),
-        scrollbar: Scrollbar::default(),
+        scrollbar: Scrollbar {
+            autohide: system.source.scrollbar_autohide(),
+            ..Scrollbar::default()
+        },
         titlebar_height,
         root_font_size,
         sizes,
@@ -339,6 +342,28 @@ mod tests {
         assert_eq!(resolved.sizes.small.text_size, px(11.05));
     }
 
+    #[test]
+    fn scrollbar_autohide_follows_the_platform_source() {
+        let mut stated = system(ThemeVariant::Dark, PartialPalette::default(), None);
+        stated.source = SourceKind::Macos;
+        assert!(resolve(&stated, None, None).scrollbar.autohide);
+
+        stated.source = SourceKind::Gnome;
+        assert!(resolve(&stated, None, None).scrollbar.autohide);
+
+        stated.source = SourceKind::Omarchy;
+        assert!(resolve(&stated, None, None).scrollbar.autohide);
+
+        stated.source = SourceKind::Windows;
+        assert!(!resolve(&stated, None, None).scrollbar.autohide);
+
+        stated.source = SourceKind::Kde;
+        assert!(!resolve(&stated, None, None).scrollbar.autohide);
+
+        stated.source = SourceKind::Fallback;
+        assert!(!resolve(&stated, None, None).scrollbar.autohide);
+    }
+
     #[test]
     fn a_stated_input_height_overrides_the_derived_one_and_scales() {
         let mut stated = system(ThemeVariant::Light, PartialPalette::default(), None);