Fix scrollbar regression

lib/ui/src/layout.rs

@@ -313,9 +313,11 @@ fn layout_element(
                 perf_stats,
             );
             let provisional_content_height = content_size.height.max(viewport_rect.size.height);
+            let requested_offset_y = scroll_box.offset_y.max(0.0);
             let provisional_max_offset_y =
                 (provisional_content_height - viewport_rect.size.height).max(0.0);
-            let mut offset_y = scroll_box.offset_y.max(0.0).min(provisional_max_offset_y);
+            let mut offset_y = requested_offset_y.min(provisional_max_offset_y);
+            let child_scene_start = scene.items.len();
 
             if viewport_rect.size.width > 0.0 && viewport_rect.size.height > 0.0 {
                 scene.push_clip(viewport_rect, 0.0);
@@ -345,7 +347,31 @@ fn layout_element(
                 .max(actual_content_height.ceil())
                 .max(viewport_rect.size.height);
             let max_offset_y = (content_height - viewport_rect.size.height).max(0.0);
-            offset_y = offset_y.min(max_offset_y);
+            let corrected_offset_y = requested_offset_y.min(max_offset_y);
+            if (corrected_offset_y - offset_y).abs() > f32::EPSILON
+                && viewport_rect.size.width > 0.0
+                && viewport_rect.size.height > 0.0
+            {
+                offset_y = corrected_offset_y;
+                scene.items.truncate(child_scene_start);
+                scene.push_clip(viewport_rect, 0.0);
+                interaction.children = layout_container_children(
+                    element,
+                    Rect::new(
+                        viewport_rect.origin.x,
+                        viewport_rect.origin.y - offset_y,
+                        viewport_rect.size.width,
+                        content_height,
+                    ),
+                    &interaction.path,
+                    scene,
+                    text_system,
+                    perf_stats,
+                );
+                scene.pop_clip();
+            } else {
+                offset_y = corrected_offset_y;
+            }
             interaction.scroll_metrics = Some(ScrollMetrics {
                 viewport_rect,
                 content_height,
@@ -1701,6 +1727,41 @@ mod tests {
         assert!(text_bottom >= viewport_bottom - 1.0);
     }
 
+    #[test]
+    fn scroll_box_preserves_requested_offset_for_direct_text_children() {
+        let scrollbox_id = ElementId::new(23);
+        let root = Element::column().child(
+            Element::scroll_box(240.0)
+                .id(scrollbox_id)
+                .width(320.0)
+                .height(120.0)
+                .padding(Edges::all(8.0))
+                .child(Element::text(
+                    "line 01\nline 02\nline 03\nline 04\nline 05\nline 06\nline 07\nline 08\nline 09\nline 10\nline 11\nline 12\nline 13\nline 14\nline 15\nline 16\nline 17\nline 18\nline 19\nline 20\nline 21\nline 22\nline 23\nline 24\nline 25\nline 26",
+                    TextStyle::new(16.0, Color::rgb(0xFF, 0xFF, 0xFF)).with_line_height(20.0),
+                )),
+        );
+
+        let snapshot = layout_snapshot(1, UiSize::new(360.0, 220.0), &root);
+        let scroll_metrics = snapshot
+            .interaction_tree
+            .scroll_metrics_for_element(scrollbox_id)
+            .expect("scroll box should expose scroll metrics");
+        let visible_text = snapshot
+            .scene
+            .items
+            .iter()
+            .find_map(|item| match item {
+                DisplayItem::Text(text) => Some(text),
+                _ => None,
+            })
+            .expect("scroll box should emit direct text children");
+
+        assert!(scroll_metrics.max_offset_y > 240.0);
+        assert_eq!(scroll_metrics.offset_y, 240.0);
+        assert!(visible_text.origin.y < scroll_metrics.viewport_rect.origin.y);
+    }
+
     #[test]
     fn interaction_tree_hit_test_returns_deepest_pointer_target() {
         let root = Element::column()