--- a/gfx/wr/webrender/src/picture.rs
+++ b/gfx/wr/webrender/src/picture.rs
@@ -1444,16 +1444,18 @@ pub struct TileCacheInstance {
     /// List of opacity bindings, with some extra information
     /// about whether they changed since last frame.
     opacity_bindings: FastHashMap<PropertyBindingId, OpacityBindingInfo>,
     /// Switch back and forth between old and new bindings hashmaps to avoid re-allocating.
     old_opacity_bindings: FastHashMap<PropertyBindingId, OpacityBindingInfo>,
     /// List of spatial nodes, with some extra information
     /// about whether they changed since last frame.
     spatial_nodes: FastHashMap<SpatialNodeIndex, SpatialNodeDependency>,
+    /// Switch back and forth between old and new spatial nodes hashmaps to avoid re-allocating.
+    old_spatial_nodes: FastHashMap<SpatialNodeIndex, SpatialNodeDependency>,
     /// A set of spatial nodes that primitives / clips depend on found
     /// during dependency creation. This is used to avoid trying to
     /// calculate invalid relative transforms when building the spatial
     /// nodes hash above.
     used_spatial_nodes: FastHashSet<SpatialNodeIndex>,
     /// The current dirty region tracker for this picture.
     pub dirty_region: DirtyRegion,
     /// Current size of tiles in picture units.
@@ -1518,16 +1520,17 @@ impl TileCacheInstance {
             ),
             map_child_pic_to_surface: SpaceMapper::new(
                 ROOT_SPATIAL_NODE_INDEX,
                 PictureRect::zero(),
             ),
             opacity_bindings: FastHashMap::default(),
             old_opacity_bindings: FastHashMap::default(),
             spatial_nodes: FastHashMap::default(),
+            old_spatial_nodes: FastHashMap::default(),
             used_spatial_nodes: FastHashSet::default(),
             dirty_region: DirtyRegion::new(),
             tile_size: PictureSize::zero(),
             tile_rect: TileRect::zero(),
             tile_bounds_p0: TileOffset::zero(),
             tile_bounds_p1: TileOffset::zero(),
             local_rect: PictureRect::zero(),
             local_clip_rect: PictureRect::zero(),
@@ -2236,31 +2239,32 @@ impl TileCacheInstance {
             .into();
         let root_transform_changed = root_transform != self.root_transform;
         if root_transform_changed {
             self.root_transform = root_transform;
             frame_state.composite_state.dirty_rects_are_valid = false;
         }
 
         // Diff the state of the spatial nodes between last frame build and now.
-        let mut old_spatial_nodes = mem::replace(&mut self.spatial_nodes, FastHashMap::default());
+        mem::swap(&mut self.spatial_nodes, &mut self.old_spatial_nodes);
 
         // TODO(gw): Maybe remove the used_spatial_nodes set and just mutate / create these
         //           diffs inside add_prim_dependency?
+        self.spatial_nodes.clear();
         for spatial_node_index in self.used_spatial_nodes.drain() {
             // Get the current relative transform.
             let mut value = get_transform_key(
                 spatial_node_index,
                 self.spatial_node_index,
                 frame_context.clip_scroll_tree,
             );
 
             // Check if the transform has changed from last frame
             let mut changed = true;
-            if let Some(old_info) = old_spatial_nodes.remove(&spatial_node_index) {
+            if let Some(old_info) = self.old_spatial_nodes.remove(&spatial_node_index) {
                 if old_info.value == value {
                     // Since the transform key equality check applies epsilon, if we
                     // consider the value to be the same, store that old value to avoid
                     // missing very slow drifts in the value over time.
                     // TODO(gw): We should change ComparableVec to use a trait for comparison
                     //           rather than PartialEq.
                     value = old_info.value;
                     changed = false;