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,
         }
     }
 }