chore: reduce component overhead (#3)

Reviewed-on: #3

crates/core/tests/scrollbar.rs

@@ -38,7 +38,7 @@ impl Render for UniformShell {
                     .h_full()
                     .w_full(),
                 )
-                .child(Scrollbar::vertical(self.handle.clone())),
+                .child(Scrollbar::vertical("rows", self.handle.clone())),
         )
     }
 }
@@ -64,7 +64,7 @@ impl Render for ListShell {
                     .h_full()
                     .w_full(),
                 )
-                .child(Scrollbar::vertical(self.state.clone())),
+                .child(Scrollbar::vertical("rows", self.state.clone())),
         )
     }
 }
@@ -271,3 +271,99 @@ fn dragging_the_thumb_drives_a_list_state_and_reports_the_drag() {
         "the drag ends with the mouse"
     );
 }
+
+struct TwoShell {
+    left: UniformListScrollHandle,
+    right: UniformListScrollHandle,
+}
+
+impl Render for TwoShell {
+    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+        div()
+            .size_full()
+            .flex()
+            .flex_row()
+            .child(
+                div()
+                    .relative()
+                    .h(px(VIEWPORT_HEIGHT))
+                    .w(px(300.))
+                    .child(
+                        uniform_list("left-rows", ROWS, |range, _, _| {
+                            range
+                                .map(|ix| div().h(px(ROW_HEIGHT)).child(format!("row {ix}")))
+                                .collect()
+                        })
+                        .track_scroll(&self.left)
+                        .h_full()
+                        .w_full(),
+                    )
+                    .child(Scrollbar::vertical("left", self.left.clone())),
+            )
+            .child(
+                div()
+                    .relative()
+                    .h(px(VIEWPORT_HEIGHT))
+                    .w(px(300.))
+                    .child(
+                        uniform_list("right-rows", ROWS, |range, _, _| {
+                            range
+                                .map(|ix| div().h(px(ROW_HEIGHT)).child(format!("row {ix}")))
+                                .collect()
+                        })
+                        .track_scroll(&self.right)
+                        .h_full()
+                        .w_full(),
+                    )
+                    .child(Scrollbar::vertical("right", self.right.clone())),
+            )
+    }
+}
+
+fn two_shell() -> (
+    TestAppContext,
+    VisualTestContext,
+    UniformListScrollHandle,
+    UniformListScrollHandle,
+) {
+    let mut cx = TestAppContext::single();
+    cx.update(init);
+    let (left, right) = (
+        UniformListScrollHandle::new(),
+        UniformListScrollHandle::new(),
+    );
+    let (left_for_view, right_for_view) = (left.clone(), right.clone());
+    let window = cx.add_window(move |_, _| TwoShell {
+        left: left_for_view,
+        right: right_for_view,
+    });
+    let mut visual = VisualTestContext::from_window(window.into(), &cx);
+    visual.update(|window, cx| {
+        window.draw(cx).clear(cx);
+    });
+    (cx, visual, left, right)
+}
+
+/// Dragging one scrollbar's thumb must never move a sibling scrollbar: the
+/// drag state is keyed per scrollbar id, not shared across instances.
+#[test]
+fn sibling_scrollbars_drag_independently() {
+    let (cx, mut visual, left, right) = two_shell();
+    let metrics = Metrics::measure(&cx, &left);
+    assert!(metrics.max_offset > px(0.));
+
+    press(&mut visual, metrics.thumb_center());
+    drag_to(&mut visual, metrics.full_drag_target());
+    release(&mut visual, metrics.full_drag_target());
+
+    let offset = left.offset().y;
+    assert!(
+        (offset.abs() - metrics.max_offset).abs() < px(0.5),
+        "the left drag reaches the end, got {offset}"
+    );
+    assert_eq!(
+        right.offset().y,
+        px(0.),
+        "the left drag leaked into the right scrollbar"
+    );
+}