Replace SnapshotSubviews method with ViewCollectionHelpers class

Author: BDisp

Terminal.Gui/ViewBase/View.Hierarchy.cs

@@ -36,8 +36,6 @@ public View? SuperView
         private set => SetSuperView (value);
     }
 
-    internal List<View> SnapshotSubviews () { return [..InternalSubViews]; }
-
     private void SetSuperView (View? value)
     {
         if (_superView == value)
@@ -560,3 +558,22 @@ private void PerformActionForSubView (View subview, Action<View> action)
 
     #endregion SubViewOrdering
 }
+
+internal static class ViewCollectionHelpers
+{
+    /// <summary>Returns a defensive copy of any <see cref="IEnumerable{T}"/>.</summary>
+    internal static View [] Snapshot (this IEnumerable<View> source)
+    {
+        if (source is IList<View> list)
+        {
+            // The list parameter might be the live `_subviews`, so freeze it under a lock
+            lock (list)
+            {
+                return [.. list]; // C# 12 slice copy (= new List<View>(list).ToArray())
+            }
+        }
+
+        // Anything else (LINQ result, iterator block, etc.) we just enumerate.
+        return source.ToArray (); // Safe because it’s not shared mutable state
+    }
+}