Fix ordering of transforms on a reference frame.

webrender/src/spatial_node.rs

@@ -256,12 +256,20 @@ impl SpatialNode {
                     .pre_mul(&source_transform.into())
                     .pre_mul(&info.source_perspective);
 
-                // The transformation for this viewport in world coordinates is the transformation for
-                // our parent reference frame, plus any accumulated scrolling offsets from nodes
-                // between our reference frame and this node. Finally, we also include
-                // whatever local transformation this reference frame provides.
+                // In order to compute a transformation to world coordinates, we need to apply the
+                // following transforms in order:
+                //   state.parent_accumulated_scroll_offset
+                //   info.source_perspective
+                //   info.source_transform
+                //   info.origin_in_parent_reference_frame
+                //   state.parent_reference_frame_transform
+                // The first one incorporates the scrolling effect of any scrollframes/sticky nodes
+                // between this reference frame and the parent reference frame. The middle three
+                // transforms (which are combined into info.resolved_transform) do the conversion
+                // into the parent reference frame's coordinate space, and then the last one
+                // applies the parent reference frame's transform to the world space.
                 let relative_transform = info.resolved_transform
-                    .post_translate(state.parent_accumulated_scroll_offset)
+                    .pre_translate(&state.parent_accumulated_scroll_offset)
                     .to_transform()
                     .with_destination::<LayoutPixel>();
                 self.world_viewport_transform =

webrender/src/util.rs

@@ -540,6 +540,7 @@ impl<Src, Dst> FastTransform<Src, Dst> {
         }
     }
 
+    #[allow(dead_code)]
     pub fn post_translate(&self, new_offset: TypedVector2D<f32, Dst>) -> Self {
         match *self {
             FastTransform::Offset(offset) => {