Use real clips when generating scroll roots

This is the first step toward removing inherited clips in favor of
scroll roots for handling overflow and CSS clipping. This will allow us
to more easily handle elements that should not be clipped. While we are
still using inherited clips here, we now properly clip some types of
content that wasn't clipped before.

components/layout/display_list_builder.rs

@@ -520,7 +520,9 @@ fn handle_overlapping_radii(size: &Size2D<Au>, radii: &BorderRadii<Au>) -> Borde
     }
 }
 
-fn build_border_radius(abs_bounds: &Rect<Au>, border_style: &style_structs::Border) -> BorderRadii<Au> {
+fn build_border_radius(abs_bounds: &Rect<Au>,
+                       border_style: &style_structs::Border)
+                       -> BorderRadii<Au> {
     // TODO(cgaebel): Support border radii even in the case of multiple border widths.
     // This is an extension of supporting elliptical radii. For now, all percentage
     // radii will be relative to the width.
@@ -537,6 +539,34 @@ fn build_border_radius(abs_bounds: &Rect<Au>, border_style: &style_structs::Bord
     })
 }
 
+/// Get the border radius for the rectangle inside of a rounded border. This is useful
+/// for building the clip for the content inside the border.
+fn build_border_radius_for_inner_rect(outer_rect: &Rect<Au>,
+                                      style: Arc<ServoComputedValues>)
+                                      -> BorderRadii<Au> {
+    let mut radii = build_border_radius(&outer_rect, style.get_border());
+    if radii.is_square() {
+        return radii;
+    }
+
+    // Since we are going to using the inner rectangle (outer rectangle minus
+    // border width), we need to adjust to border radius so that we are smaller
+    // rectangle with the same border curve.
+    let border_widths = style.logical_border_width().to_physical(style.writing_mode);
+    radii.top_left.width = cmp::max(Au(0), radii.top_left.width - border_widths.left);
+    radii.bottom_left.width = cmp::max(Au(0), radii.bottom_left.width - border_widths.left);
+
+    radii.top_right.width = cmp::max(Au(0), radii.top_right.width - border_widths.right);
+    radii.bottom_right.width = cmp::max(Au(0), radii.bottom_right.width - border_widths.right);
+
+    radii.top_left.height = cmp::max(Au(0), radii.top_left.height - border_widths.top);
+    radii.top_right.height = cmp::max(Au(0), radii.top_right.height - border_widths.top);
+
+    radii.bottom_left.height = cmp::max(Au(0), radii.bottom_left.height - border_widths.bottom);
+    radii.bottom_right.height = cmp::max(Au(0), radii.bottom_right.height - border_widths.bottom);
+    radii
+}
+
 impl FragmentDisplayListBuilding for Fragment {
     fn build_display_list_for_background_if_applicable(&self,
                                                        state: &mut DisplayListBuildState,
@@ -1978,12 +2008,21 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
         let new_scroll_root_id = ScrollRootId::new_of_type(self.fragment.node.id() as usize,
                                                            self.fragment.fragment_type());
 
+        let clip_rect = Rect::new(Point2D::zero(), content_box.size);
+        let mut clip = ClippingRegion::from_rect(&clip_rect);
+
+        let border_radii = build_border_radius_for_inner_rect(&border_box,
+                                                              self.fragment.style.clone());
+        if !border_radii.is_square() {
+            clip.intersect_with_rounded_rect(&clip_rect, &border_radii)
+        }
+
         let content_size = self.base.overflow.scroll.origin + self.base.overflow.scroll.size;
         state.add_scroll_root(
             ScrollRoot {
                 id: new_scroll_root_id,
                 parent_id: containing_scroll_root_id,
-                clip: Rect::new(Point2D::zero(), content_box.size),
+                clip: clip,
                 content_rect: Rect::new(content_box.origin,
                                         Size2D::new(content_size.x, content_size.y)),
             },