diff --git a/crates/render/src/changed.rs b/crates/render/src/changed.rs
index c1612f0c9..1c1524703 100644
--- a/crates/render/src/changed.rs
+++ b/crates/render/src/changed.rs
@@ -183,8 +183,8 @@ impl Diff<'_, '_> {
         {
             // An Image Trace object draws its hidden source image, its first member, in some views.
             // Each version's own: an edit can put another member first (undoing Send to Back).
-            let first = |m: &[Arc<Node>]| old.extras().trace.is_some().then(|| m.first().map(|s| s.id)).flatten();
-            let outer = std::mem::replace(&mut self.sources, (first(a), first(b)));
+            let first = |n: &Node, m: &[Arc<Node>]| n.extras().trace.is_some().then(|| m.first().map(|s| s.id)).flatten();
+            let outer = std::mem::replace(&mut self.sources, (first(old, a), first(new, b)));
             self.children(a, b);
             self.sources = outer;
         } else if let Some((area, moved, anchors)) = moved_anchors(old, new) {
diff --git a/crates/render/src/lib.rs b/crates/render/src/lib.rs
index 32fb8fd02..4b2434282 100644
--- a/crates/render/src/lib.rs
+++ b/crates/render/src/lib.rs
@@ -175,7 +175,11 @@ pub struct Renderer {
     /// let go when a frame ends.
     pub(crate) pass_bounds: PtrMap<usize, (Arc<Node>, Option<Rect>)>,
     /// The 3D effects' art among the art `fx_art` made, by its address, with the art (weakly: what
-    /// keeps the art is `fx_arts` and `measured`) and its bounds once measured.
+    /// keeps the art is `fx_arts` and `measured`) and its bounds once measured. An entry is added
+    /// for every 3D art made, measured or not, and entries whose art is gone are swept when the map
+    /// reaches twice the size it kept at the last sweep (at least 1024), so it holds at most about
+    /// twice the live 3D art plus 1024 entries. Until swept, an entry's weak pointer keeps the art's
+    /// allocation (not its contents) alive.
     three_d_bounds: PtrMap<usize, ThreeDArt>,
     /// How many entries `three_d_bounds` kept when it last let go of the art no longer held: it
     /// does so again when it holds twice as many.