NativeScript
diff --git a/‎NativeScript/runtime/js/performance.js‎
Lines changed: 14 additions & 5 deletions b/‎NativeScript/runtime/js/performance.js‎
Lines changed: 14 additions & 5 deletions
diff --git a/‎TestRunner/app/shared‎ b/‎TestRunner/app/shared‎
diff --git a/‎docs/performance.md‎
Lines changed: 4 additions & 7 deletions b/‎docs/performance.md‎
Lines changed: 4 additions & 7 deletions
@@ -8,9 +8,6 @@
 // against an equivalent bag.
 //
 // Deliberate deviations from the specs:
-// - mark/measure `detail` is held by reference (this runtime has no
-//   structuredClone), so entries retain whatever the caller passed until
-//   clearMarks()/clearMeasures(); the user-timing buffers are unbounded.
 // - Observer callbacks are delivered from a microtask rather than a queued
 //   task. Delivery is still asynchronous relative to mark()/measure(), but it
 //   precedes timer callbacks scheduled in the same turn.
@@ -42,6 +39,18 @@ var g = globalThis;
 const EventTarget = g.EventTarget;
 const enqueueMicrotask = g.queueMicrotask;
 const reportException = g.reportError;
+// mark/measure `detail` is structured-cloned per spec, so an entry holds a
+// snapshot and an uncloneable detail throws DataCloneError. The identity
+// fallback keeps this file portable to a runtime that ships the Performance
+// API before structuredClone — there, detail degrades to by-reference; the
+// user-timing buffers are unbounded either way, so entries retain their
+// detail until clearMarks()/clearMeasures().
+const cloneDetail =
+  typeof g.structuredClone === "function"
+    ? g.structuredClone
+    : function (value) {
+        return value;
+      };
 
 // Construction token: interfaces whose constructors the spec marks as not
 // user-invocable accept instances only from factories inside this module.
@@ -135,7 +144,7 @@ class PerformanceMark extends PerformanceEntry {
         }
       }
       if (markOptions.detail !== undefined && markOptions.detail !== null) {
-        detail = markOptions.detail;
+        detail = cloneDetail(markOptions.detail);
       }
     }
     super(kInternal, name, "mark", startTime === undefined ? now() : startTime, 0);
@@ -516,7 +525,7 @@ class Performance extends EventTarget {
         startTime = 0;
       }
       if (o.detail !== undefined && o.detail !== null) {
-        detail = o.detail;
+        detail = cloneDetail(o.detail);
       }
     } else {
       endTime = endMark !== undefined ? convertMarkToTimestamp(endMark) : now();
 
@@ -56,13 +56,10 @@ shares `performance.timeOrigin` as its base.
 
 ## Deviations from the specs
 
-- **`detail` is held by reference.** The runtime has no `structuredClone`, so
-  `mark`/`measure` `detail` values are stored as-is. Mutating the object later
-  is visible through the entry, and entries retain whatever `detail`
-  references until `clearMarks()`/`clearMeasures()`.
-- **Buffers are unbounded.** Per spec for user timing, but combined with
-  by-reference `detail` it means a long-lived app marking in a loop should
-  clear entries periodically.
+- **Buffers are unbounded.** Per spec for user timing. `detail` is
+  structured-cloned at entry creation (per spec — an uncloneable `detail`
+  throws the `DataCloneError`-named error), so entries hold snapshots, but a
+  long-lived app marking in a loop should still clear entries periodically.
 - **Observer callbacks run from a microtask**, not a queued task: delivery is
   asynchronous relative to `mark()`/`measure()` but precedes timer callbacks
   scheduled in the same turn. Callback exceptions are routed to