Example 02 realized

lib/ui/src/layout.rs

@@ -116,6 +116,10 @@ impl InteractionTree {
     pub fn scroll_metrics_for_element(&self, element_id: ElementId) -> Option<&ScrollMetrics> {
         scroll_metrics_for_element_node(&self.root, element_id)
     }
+
+    pub fn rect_for_element(&self, element_id: ElementId) -> Option<Rect> {
+        rect_for_element_node(&self.root, element_id)
+    }
 }
 
 pub fn layout_snapshot(version: u64, logical_size: UiSize, root: &Element) -> LayoutSnapshot {
@@ -309,7 +313,9 @@ fn layout_element(
                 perf_stats,
             );
             let provisional_content_height = content_size.height.max(viewport_rect.size.height);
-            let mut 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);
 
             if viewport_rect.size.width > 0.0 && viewport_rect.size.height > 0.0 {
                 scene.push_clip(viewport_rect, 0.0);
@@ -509,6 +515,18 @@ fn scroll_metrics_for_element_node(
     None
 }
 
+fn rect_for_element_node(node: &LayoutNode, element_id: ElementId) -> Option<Rect> {
+    if node.element_id == Some(element_id) {
+        return Some(node.rect);
+    }
+    for child in &node.children {
+        if let Some(rect) = rect_for_element_node(child, element_id) {
+            return Some(rect);
+        }
+    }
+    None
+}
+
 fn point_hits_node_shape(node: &LayoutNode, point: crate::scene::Point) -> bool {
     node.rect.contains(point)
         && (node.corner_radius <= 0.0
@@ -1598,6 +1616,51 @@ mod tests {
         assert!(scroll_metrics.scrollbar_thumb.is_some());
     }
 
+    #[test]
+    fn scroll_box_clamps_stale_offset_before_laying_out_reflowed_content() {
+        let scrollbox_id = ElementId::new(19);
+        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::paragraph(
+                    "When the viewport becomes wider, wrapped scroll-box content can get much \
+                     shorter. Layout should clamp any now-invalid stale offset before positioning \
+                     the children so the viewport does not open up an empty gap at the bottom.",
+                    TextStyle::new(16.0, Color::rgb(0xFF, 0xFF, 0xFF))
+                        .with_line_height(22.0)
+                        .with_wrap(TextWrap::Word),
+                )),
+        );
+
+        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 still emit text");
+
+        let text_bounds = visible_text
+            .bounds
+            .expect("prepared text should expose bounds for wrapped content");
+        let text_bottom = visible_text.origin.y + text_bounds.height;
+        let viewport_bottom =
+            scroll_metrics.viewport_rect.origin.y + scroll_metrics.viewport_rect.size.height;
+
+        assert!(scroll_metrics.offset_y <= scroll_metrics.max_offset_y);
+        assert!(text_bottom >= viewport_bottom - 1.0);
+    }
+
     #[test]
     fn interaction_tree_hit_test_returns_deepest_pointer_target() {
         let root = Element::column()