Preserve root scrolling overflow geometry

crates/obscura-js/src/runtime.rs

@@ -3194,6 +3194,43 @@ mod tests {
         assert_eq!(after[2].as_f64(), Some(80.0));
     }
 
+    #[cfg(feature = "render")]
+    #[test]
+    fn root_overflow_clip_preserves_cssom_scroll_range() {
+        let dom = parse_html(
+            r#"<html style="margin:0;height:100%;overflow:hidden">
+                <body style="margin:0;height:100%;overflow:hidden">
+                    <main id="main" style="padding-top:48px">
+                        <div style="height:5000px"></div>
+                    </main>
+                </body>
+            </html>"#,
+        );
+        let mut rt = ObscuraJsRuntime::new();
+        rt.set_dom(dom);
+        rt.set_viewport(900.0, 1000.0);
+        rt.run_page_init();
+
+        let result = rt
+            .evaluate(
+                r#"
+                const main = document.getElementById("main");
+                const before = main.getBoundingClientRect();
+                scrollTo(0, 99999);
+                const after = main.getBoundingClientRect();
+                return [
+                    before.height,
+                    document.documentElement.scrollHeight,
+                    document.body.scrollHeight,
+                    scrollY,
+                    after.top,
+                ];
+                "#,
+            )
+            .unwrap();
+        assert_eq!(result, serde_json::json!([5048, 5048, 5048, 4048, -4048]));
+    }
+
     #[cfg(feature = "render")]
     #[tokio::test(flavor = "current_thread")]
     async fn rendered_window_scroll_events_require_actual_movement() {

crates/obscura-render/src/dom.rs

@@ -285,25 +285,22 @@ impl DomLayout {
         let mut right = viewport.0.max(0.0);
         let mut bottom = viewport.1.max(0.0);
 
-        for (&id, rect) in &self.rects {
-            if fixed.contains(&id) {
-                continue;
-            }
-            let (tx, ty) = self.translates.get(&id).copied().unwrap_or((0.0, 0.0));
-            let mut painted = Rect {
-                x: rect.x + tx,
-                y: rect.y + ty,
-                width: rect.width,
-                height: rect.height,
-            };
-            if let Some(clip) = self.clip_rects.get(&id).copied().flatten() {
-                let Some(clipped) = painted.intersect(&clip) else {
-                    continue;
-                };
-                painted = clipped;
-            }
-            right = right.max(painted.x + painted.width);
-            bottom = bottom.max(painted.y + painted.height);
+        if let Some(root) = tree
+            .descendants(tree.document())
+            .into_iter()
+            .find(|id| tree.get_node(*id).is_some_and(|node| node.is_element()))
+        {
+            accumulate_scrolling_overflow(
+                tree,
+                root,
+                None,
+                &fixed,
+                &self.rects,
+                &self.styles,
+                &self.translates,
+                &mut right,
+                &mut bottom,
+            );
         }
 
         (right.ceil(), bottom.ceil())
@@ -458,6 +455,69 @@ impl DomLayout {
     }
 }
 
+#[allow(clippy::too_many_arguments)]
+fn accumulate_scrolling_overflow(
+    tree: &DomTree,
+    id: NodeId,
+    inherited_clip: Option<Rect>,
+    fixed: &HashSet<NodeId>,
+    rects: &HashMap<NodeId, Rect>,
+    styles: &HashMap<NodeId, crate::LayoutStyle>,
+    translates: &HashMap<NodeId, (f32, f32)>,
+    right: &mut f32,
+    bottom: &mut f32,
+) {
+    if fixed.contains(&id) {
+        return;
+    }
+
+    let translated = rects.get(&id).map(|rect| {
+        let (tx, ty) = translates.get(&id).copied().unwrap_or((0.0, 0.0));
+        Rect {
+            x: rect.x + tx,
+            y: rect.y + ty,
+            width: rect.width,
+            height: rect.height,
+        }
+    });
+    if let Some(overflow) = translated {
+        let visible = if let Some(clip) = inherited_clip {
+            overflow.intersect(&clip)
+        } else {
+            Some(overflow)
+        };
+        if let Some(overflow) = visible {
+            *right = right.max(overflow.x + overflow.width);
+            *bottom = bottom.max(overflow.y + overflow.height);
+        }
+    }
+
+    // Overflow propagated from html/body establishes the viewport clip for
+    // painting, but it does not truncate the root scrolling area's CSSOM
+    // overflow dimensions. Ordinary descendant overflow clips still bound
+    // their subtree's contribution.
+    let is_viewport_overflow_source = tree.get_node(id).is_some_and(|node| {
+        node.as_element()
+            .is_some_and(|element| matches!(element.local.as_ref(), "html" | "body"))
+    });
+    let child_clip = match (styles.get(&id), rects.get(&id)) {
+        (Some(style), Some(rect)) if style.overflow_hidden && !is_viewport_overflow_source => {
+            let (tx, ty) = translates.get(&id).copied().unwrap_or((0.0, 0.0));
+            let own = overflow_clip_rect(rect, style, tx, ty);
+            Some(match inherited_clip {
+                Some(clip) => clip.intersect(&own).unwrap_or(Rect::default()),
+                None => own,
+            })
+        }
+        _ => inherited_clip,
+    };
+    for child in tree.children(id) {
+        accumulate_scrolling_overflow(
+            tree, child, child_clip, fixed, rects, styles, translates, right, bottom,
+        );
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) enum GeneratedBoxKind {
     Before,
@@ -8164,6 +8224,38 @@ mod tests {
         }
     }
 
+    #[test]
+    fn viewport_overflow_clip_does_not_truncate_root_scrolling_overflow() {
+        let tree = parse_html(
+            r#"<style>
+                html,body{margin:0;height:100%;overflow:hidden}
+                main{padding-top:48px}
+                #long{height:5000px}
+            </style><main id="main"><div id="long"></div></main>"#,
+        );
+        let laid = layout_dom(&tree, (900.0, 1000.0));
+        let main = laid.rects[&tree.get_element_by_id("main").unwrap()];
+        let content = laid.scrolling_content_size(&tree, (900.0, 1000.0));
+        assert!((main.height - 5048.0).abs() < 0.01, "{main:?}");
+        assert_eq!(content, (900.0, 5048.0));
+    }
+
+    #[test]
+    fn descendant_overflow_clip_still_bounds_root_scrolling_overflow() {
+        let tree = parse_html(
+            r#"<style>
+                html,body{margin:0}
+                #clip{height:100px;overflow:hidden}
+                #long{height:5000px}
+            </style><div id="clip"><div id="long"></div></div>"#,
+        );
+        let laid = layout_dom(&tree, (900.0, 1000.0));
+        assert_eq!(
+            laid.scrolling_content_size(&tree, (900.0, 1000.0)),
+            (900.0, 1000.0)
+        );
+    }
+
     #[test]
     fn size_container_auto_block_size_ignores_descendants() {
         let tree = parse_html(