layout: Fix incorrect overflow clipping logic for the root flow.

Part of a fix for Google search results. Closes #13932.
servo · pcwalton · Oct 27, 2016 · Oct 27, 2016 · Oct 27, 2016 · Oct 27, 2016
commit 53c4845e2109e90df08937bcc62120a5b23688e4
diff --git a/components/layout/display_list_builder.rs b/components/layout/display_list_builder.rs
@@ -57,6 +57,7 @@ use style::values::specified::{AngleOrCorner, HorizontalDirection, VerticalDirec
 use style_traits::cursor::Cursor;
 use table_cell::CollapsedBordersForCell;
 use url::Url;
+use util::geometry;
 use util::opts;
 
 static THREAD_TINT_COLORS: [Color; 8] = [
@@ -1367,7 +1368,10 @@ impl FragmentDisplayListBuilding for Fragment {
                                                           .relative_containing_block_mode,
                                                CoordinateSystem::Parent);
         let overflow = if scrolls_overflow_area {
-            Rect::new(Point2D::zero(), base_flow.overflow.scroll.size)
+            base_flow.overflow
+                     .scroll
+                     .intersection(&Rect::new(Point2D::zero(), geometry::max_rect().size))
+                     .unwrap_or(Rect::zero())
         } else {
             // First, compute the offset of our border box (including relative positioning)
             // from our flow origin, since that is what `BaseFlow::overflow` is relative to.

diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
@@ -928,18 +928,21 @@ impl LayoutThread {
 
                         debug!("Done building display list.");
 
-                        let root_size = {
+                        let root_bounds = {
                             let root_flow = flow::base(layout_root);
                             if rw_data.stylist.viewport_constraints().is_some() {
-                                root_flow.position.size.to_physical(root_flow.writing_mode)
+                                root_flow.position.to_physical(root_flow.writing_mode,
+                                                               Size2D::zero())
                             } else {
-                                root_flow.overflow.scroll.size
+                                root_flow.overflow.scroll
                             }
                         };
 
-                        let origin = Rect::new(Point2D::new(Au(0), Au(0)), root_size);
-                        root_stacking_context.bounds = origin;
-                        root_stacking_context.overflow = origin;
+                        let root_bounds =
+                            root_bounds.intersection(&Rect::new(Point2D::zero(), max_rect().size))
+                                       .unwrap_or(Rect::zero());
+                        root_stacking_context.bounds = root_bounds;
+                        root_stacking_context.overflow = root_bounds;
 
                         rw_data.display_list =
                             Some(Arc::new(DisplayList::new(root_stacking_context,

diff --git a/tests/html/body_overflow_size.html b/tests/html/body_overflow_size.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+This page should not be scrollable.
+<div style="position: absolute; top: -1000em;">X</div>
+