feat: add scrollbar for virtualized lists

crates/theme/src/lib.rs

@@ -225,7 +225,8 @@ impl Default for Motion {
     }
 }
 
-/// Overlay scrollbar geometry, transcribed from the platform's overlay crollbars.
+/// Overlay scrollbar presentation, transcribed from the platform's overlay
+/// crollbars.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct Scrollbar {
     /// The painted thumb's thickness.
@@ -234,6 +235,8 @@ pub struct Scrollbar {
     pub inset: Pixels,
     /// The thumb never shrinks below this length, so it stays grabbable.
     pub min_thumb: Pixels,
+    /// Whether the platform's scrollbars hide while idle.
+    pub autohide: bool,
 }
 
 impl Default for Scrollbar {
@@ -242,6 +245,7 @@ impl Default for Scrollbar {
             width: px(8.),
             inset: px(2.),
             min_thumb: px(24.),
+            autohide: false,
         }
     }
 }

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);

crates/theme/src/system.rs

@@ -36,6 +36,13 @@ impl SourceKind {
             Self::Windows => "Windows 11 (UISettings)",
         }
     }
+
+    /// Overlay scrollbars hide while idle on macOS, GNOME, and Omarchy.
+    ///
+    /// Windows and KDE keep theirs drawn.
+    pub fn scrollbar_autohide(self) -> bool {
+        matches!(self, Self::Macos | Self::Omarchy | Self::Gnome)
+    }
 }
 
 /// A partial palette with optional colors.