Bug 1604280 - Fix a bug in SpatialTree::get_world_transform_impl(). r=gw

The first branch (where the node is in the root coordinate system)
was not handling the TransformScroll::Unscrolled case, which should
use the scale/offset from the viewport transform rather than the
content transform.

Differential Revision: https://phabricator.services.mozilla.com/D196692

gfx/wr/webrender/src/spatial_tree.rs

@@ -1077,7 +1077,10 @@ impl SpatialTree {
             if index == self.root_reference_frame_index {
                 CoordinateSpaceMapping::Local
             } else {
-                CoordinateSpaceMapping::ScaleOffset(child.content_transform)
+              match scroll {
+                TransformScroll::Scrolled => CoordinateSpaceMapping::ScaleOffset(child.content_transform),
+                TransformScroll::Unscrolled => CoordinateSpaceMapping::ScaleOffset(child.viewport_transform),
+              }
             }
         } else {
             let system = &self.coord_systems[child.coordinate_system_id.0 as usize];
@@ -2004,3 +2007,38 @@ fn test_find_scroll_root_2d_scale() {
 
     assert_eq!(st.find_scroll_root(sub_scroll), sub_scroll);
 }
+
+#[test]
+fn test_world_transforms() {
+  // Create a spatial tree with a scroll frame node with scroll offset (0, 200).
+  let mut cst = SceneSpatialTree::new();
+  let pid = PipelineInstanceId::new(0);
+  let scroll = cst.add_scroll_frame(
+      cst.root_reference_frame_index(),
+      ExternalScrollId(1, PipelineId::dummy()),
+      PipelineId::dummy(),
+      &LayoutRect::from_size(LayoutSize::new(400.0, 400.0)),
+      &LayoutSize::new(400.0, 800.0),
+      ScrollFrameKind::Explicit, 
+      LayoutVector2D::new(0.0, 200.0),
+      APZScrollGeneration::default(),
+      HasScrollLinkedEffect::No,
+      SpatialNodeUid::external(SpatialTreeItemKey::new(0, 1), PipelineId::dummy(), pid));
+
+  let mut st = SpatialTree::new();
+  st.apply_updates(cst.end_frame_and_get_pending_updates());
+  st.update_tree(&SceneProperties::new());
+
+  // The node's world transform should reflect the scroll offset,
+  // e.g. here it should be (0, -200) to reflect that the content has been
+  // scrolled up by 200px.
+  assert_eq!(
+      st.get_world_transform(scroll).into_transform(),
+      LayoutToWorldTransform::translation(0.0, -200.0, 0.0));
+
+  // The node's world viewport transform only reflects enclosing scrolling
+  // or transforms. Here we don't have any, so it should be the identity.
+  assert_eq!(
+      st.get_world_viewport_transform(scroll).into_transform(),
+      LayoutToWorldTransform::identity());
+}