Bug 1874199 - Split the handling of StickyFrame and ScrollFrame in SpatialNode::update_transform(). r=gw

The two cases share some code, but the next patch will add extra logic
specific to the StickyFrame case, which makes it cleaner to split them.

Differential Revision: https://phabricator.services.mozilla.com/D208236

gfx/wr/webrender/src/spatial_node.rs

@@ -518,17 +518,29 @@ impl SpatialNode {
                 self.viewport_transform = cs_scale_offset;
                 self.content_transform = cs_scale_offset;
             }
-            _ => {
-                // We calculate this here to avoid a double-borrow later.
-                let sticky_offset = self.calculate_sticky_offset(
+            SpatialNodeType::StickyFrame(ref mut info) => {
+                let sticky_offset = Self::calculate_sticky_offset(
                     &state.nearest_scrolling_ancestor_offset,
                     &state.nearest_scrolling_ancestor_viewport,
+                    info,
                 );
 
                 // The transformation for the bounds of our viewport is the parent reference frame
                 // transform, plus any accumulated scroll offset from our parents, plus any offset
                 // provided by our own sticky positioning.
                 let accumulated_offset = state.parent_accumulated_scroll_offset + sticky_offset;
+                self.viewport_transform = state.coordinate_system_relative_scale_offset
+                    .offset(snap_offset(accumulated_offset, state.coordinate_system_relative_scale_offset.scale).to_untyped());
+                self.content_transform = self.viewport_transform;
+
+                info.current_offset = sticky_offset;
+
+                self.coordinate_system_id = state.current_coordinate_system_id;
+            }
+            SpatialNodeType::ScrollFrame(_) => {
+                // The transformation for the bounds of our viewport is the parent reference frame
+                // transform, plus any accumulated scroll offset from our parents.
+                let accumulated_offset = state.parent_accumulated_scroll_offset;
                 self.viewport_transform = state.coordinate_system_relative_scale_offset
                     .offset(snap_offset(accumulated_offset, state.coordinate_system_relative_scale_offset.scale).to_untyped());
 
@@ -538,12 +550,8 @@ impl SpatialNode {
                 self.content_transform = state.coordinate_system_relative_scale_offset
                     .offset(snap_offset(added_offset, state.coordinate_system_relative_scale_offset.scale).to_untyped());
 
-                if let SpatialNodeType::StickyFrame(ref mut info) = self.node_type {
-                    info.current_offset = sticky_offset;
-                }
-
                 self.coordinate_system_id = state.current_coordinate_system_id;
-            }
+          }
         }
 
         //TODO: remove the field entirely?
@@ -555,15 +563,10 @@ impl SpatialNode {
     }
 
     fn calculate_sticky_offset(
-        &self,
         viewport_scroll_offset: &LayoutVector2D,
         viewport_rect: &LayoutRect,
+        info: &StickyFrameInfo
     ) -> LayoutVector2D {
-        let info = match self.node_type {
-            SpatialNodeType::StickyFrame(ref info) => info,
-            _ => return LayoutVector2D::zero(),
-        };
-
         if info.margins.top.is_none() && info.margins.bottom.is_none() &&
             info.margins.left.is_none() && info.margins.right.is_none() {
             return LayoutVector2D::zero();