Fix WithinAsync timeout not propagating to EventFilter across async boundaries

src/core/Akka.TestKit/TestKitBase.cs

@@ -28,6 +28,10 @@ namespace Akka.TestKit
     /// </summary>
     public abstract partial class TestKitBase : IActorRefFactory
     {
+        // AsyncLocal for proper timeout propagation across async boundaries.
+        // This ensures WithinAsync timeout flows correctly to EventFilter and other async operations.
+        private readonly AsyncLocal<TimeSpan?> _asyncLocalEnd = new();
+
         private class TestState
         {
             public TestState()
@@ -445,9 +449,21 @@ public TimeSpan Remaining
         {
             get
             {
+                // Check AsyncLocal first (async context takes precedence)
+                var asyncEnd = _asyncLocalEnd.Value;
+                if (asyncEnd.HasValue)
+                {
+                    if (asyncEnd < TimeSpan.Zero)
+                        throw new InvalidOperationException($"End can not be negative, was: {asyncEnd}");
+
+                    var asyncRemaining = asyncEnd.Value - Now;
+                    return asyncRemaining < TimeSpan.Zero ? TimeSpan.Zero : asyncRemaining;
+                }
+
+                // Fallback to instance field
                 if(_testState.End is null)
                     throw new InvalidOperationException(@"Remaining may not be called outside of ""within""");
-                
+
                 if (_testState.End < TimeSpan.Zero)
                     throw new InvalidOperationException($"End can not be negative, was: {_testState.End}");
 
@@ -466,6 +482,18 @@ public TimeSpan Remaining
         /// <returns>TBD</returns>
         protected TimeSpan RemainingOr(TimeSpan duration)
         {
+            // Check AsyncLocal first (async context takes precedence for proper timeout propagation)
+            var asyncEnd = _asyncLocalEnd.Value;
+            if (asyncEnd.HasValue)
+            {
+                if (asyncEnd < TimeSpan.Zero)
+                    throw new InvalidOperationException($"End can not be negative, was: {asyncEnd}");
+
+                var asyncRemaining = asyncEnd.Value - Now;
+                return asyncRemaining < TimeSpan.Zero ? TimeSpan.Zero : asyncRemaining;
+            }
+
+            // Fallback to instance field for backward compatibility with sync code paths
             if (!_testState.End.HasValue) return duration;
             if (_testState.End < TimeSpan.Zero)
                 throw new InvalidOperationException($"End can not be negative, was: {_testState.End}");

src/core/Akka.TestKit/TestKitBase_Within.cs

@@ -293,7 +293,10 @@ public async Task<T> WithinAsync<T>(
 
             var maxDiff = max.Min(rem);
             var prevEnd = _testState.End;
+            var prevAsyncEnd = _asyncLocalEnd.Value; // Save previous AsyncLocal value for nesting support
+
             _testState.End = start + maxDiff;
+            _asyncLocalEnd.Value = start + maxDiff; // Set AsyncLocal for proper async propagation
 
             T ret = default;
             using (var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken))
@@ -320,6 +323,7 @@ public async Task<T> WithinAsync<T>(
                     // Make sure we stop the delay task
                     cts.Cancel();
                     _testState.End = prevEnd;
+                    _asyncLocalEnd.Value = prevAsyncEnd; // Restore previous AsyncLocal value
                 }
             }