diff --git a/Sources/PortalSplitDividerCacheInvalidator.swift b/Sources/PortalSplitDividerCacheInvalidator.swift
index a9bf69a600f2..2fbdf43c1647 100644
--- a/Sources/PortalSplitDividerCacheInvalidator.swift
+++ b/Sources/PortalSplitDividerCacheInvalidator.swift
@@ -10,13 +10,12 @@ private typealias PortalSubviewComparator = @convention(c) (
 private extension NSView {
     @objc(cmux_portalAddSubview:)
     func cmux_portalAddSubview(_ subview: NSView) {
-        let previousWindow = subview.window
-        cmux_portalAddSubview(subview)
-        PortalViewHierarchyMutationTracker.recordInsertion(
+        PortalViewHierarchyMutationTracker.prepareForInsertion(
             parentView: self,
             insertedView: subview,
-            previousWindow: previousWindow
+            previousWindow: subview.window
         )
+        cmux_portalAddSubview(subview)
     }
 
     @objc(cmux_portalAddSubview:positioned:relativeTo:)
@@ -25,13 +24,12 @@ private extension NSView {
         positioned place: NSWindow.OrderingMode,
         relativeTo otherView: NSView?
     ) {
-        let previousWindow = subview.window
-        cmux_portalAddSubview(subview, positioned: place, relativeTo: otherView)
-        PortalViewHierarchyMutationTracker.recordInsertion(
+        PortalViewHierarchyMutationTracker.prepareForInsertion(
             parentView: self,
             insertedView: subview,
-            previousWindow: previousWindow
+            previousWindow: subview.window
         )
+        cmux_portalAddSubview(subview, positioned: place, relativeTo: otherView)
     }
 
     @objc(cmux_portalSetSubviews:)
diff --git a/Sources/PortalViewHierarchyMutationTracker.swift b/Sources/PortalViewHierarchyMutationTracker.swift
index c612d7d972ef..55185e1a9e82 100644
--- a/Sources/PortalViewHierarchyMutationTracker.swift
+++ b/Sources/PortalViewHierarchyMutationTracker.swift
@@ -63,10 +63,12 @@ final class PortalViewHierarchyMutationTracker: NSObject {
         return registration
     }
 
-    /// Records an insertion after AppKit has attached the child. Unknown
-    /// prebuilt subtrees fail closed, while a leaf under an indexed no-split
-    /// branch stays on the fast path without a full cache rebuild.
-    static func recordInsertion(
+    /// Updates the hierarchy proof before handing the borrowed child reference
+    /// to AppKit. SwiftUI may consume that reference during insertion, so the
+    /// swizzled boundary must not access it after the original implementation
+    /// returns. Unknown prebuilt subtrees fail closed, while a leaf under an
+    /// indexed no-split branch stays on the fast path.
+    static func prepareForInsertion(
         parentView: NSView,
         insertedView: NSView,
         previousWindow: NSWindow?