From 0860c7e03d451d1476b58aeac47b6a1100b32f16 Mon Sep 17 00:00:00 2001
From: tylerjenningsw <tylerjennings2@outlook.com>
Date: Tue, 20 Oct 2026 10:30:59 -0400
Subject: [PATCH] Expressions: an expression can't delete a kept layer's fields

---
 crates/expr/src/prelude.js | 9 +++++----
 crates/expr/src/tests.rs   | 2 ++
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/crates/expr/src/prelude.js b/crates/expr/src/prelude.js
index e525d70e3..4e532a3e9 100644
--- a/crates/expr/src/prelude.js
+++ b/crates/expr/src/prelude.js
@@ -550,12 +550,13 @@ function Layer(c, l) {
   delete f.name;
   delete f.length;
   Object.setPrototypeOf(f, Layer.prototype);
-  // Kept across evaluations (`__layer`), so an expression can't change which layer it is or leave
-  // its own fields on it. `__i` and `__s` stay writable: `__info` reads them again in an evaluation
-  // whose `__S` isn't `__s`, and every evaluation has a new `__S`.
+  // Kept across evaluations (`__layer`), so an expression can't change which layer it is, delete
+  // a field or leave its own on it. `__i` and `__s` stay writable: `__info` reads them again in an
+  // evaluation whose `__S` isn't `__s`, and every evaluation has a new `__S`.
   Object.defineProperty(f, '__c', { value: c });
   Object.defineProperty(f, '__l', { value: l });
-  f.__i = null; f.__s = null;
+  Object.defineProperty(f, '__i', { value: null, writable: true });
+  Object.defineProperty(f, '__s', { value: null, writable: true });
   Object.preventExtensions(f);
   return f;
 }
diff --git a/crates/expr/src/tests.rs b/crates/expr/src/tests.rs
index 629884e91..f54480f08 100644
--- a/crates/expr/src/tests.rs
+++ b/crates/expr/src/tests.rs
@@ -1402,6 +1402,8 @@ fn a_kept_layer_reads_the_project_again() {
     assert_close!(f.num("thisLayer.mine === undefined ? 1 : 0", 0.0), 1.0);
     let _ = f.ev(f.a, ROT, "var L = thisLayer; L.__c = 'x'; L.__l = 99; L.__i = ['Z', 7]; L.__s = __S; 0", 0.0);
     assert_close!(f.num("thisLayer.index * 10 + thisLayer.name.length", 0.0), 21.0);
+    let _ = f.ev(f.a, ROT, "var L = thisLayer; delete L.__c; delete L.__l; delete L.__i; delete L.__s; 0", 0.0);
+    assert_close!(f.num("thisLayer.index * 10 + thisLayer.name.length", 0.0), 21.0);
 }
 
 mod property_group;