Skip over transparent non-user continuations

Author: VSadov

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.CoreCLR.cs

@@ -607,8 +607,16 @@ public static void HandleSuspended<T, TOps>(T task) where T : Task, ITaskComplet
                         // the continuation chain builds from the innermost frame out and at the time when the
                         // notifier is created we do not know yet if the caller wants to continue on a context.
                         ValueTaskSourceOnCompletedFlags configFlags = ValueTaskSourceOnCompletedFlags.None;
-                        ContinuationFlags continuationFlags = headContinuation.Next!.Flags;
 
+                        // skip to the next non-transparent continuation, if such exists
+                        Continuation nextUserContinuaton = headContinuation;
+                        while ((nextUserContinuaton.Flags & continueFlags) == 0 &&
+                             nextUserContinuaton.Next != null)
+                        {
+                            nextUserContinuaton = nextUserContinuaton.Next;
+                        }
+
+                        ContinuationFlags continuationFlags = nextUserContinuaton.Flags;
                         const ContinuationFlags continueOnContextFlags =
                             ContinuationFlags.ContinueOnCapturedSynchronizationContext |
                             ContinuationFlags.ContinueOnCapturedTaskScheduler;
@@ -620,7 +628,7 @@ public static void HandleSuspended<T, TOps>(T task) where T : Task, ITaskComplet
                         }
 
                         // Clear continuation flags, so that continuation runs transparently
-                        headContinuation.Next!.Flags &= ~continueFlags;
+                        nextUserContinuaton.Flags &= ~continueFlags;
                         TOps.ValueTaskSourceOnCompleted(task, vtsNotifier, configFlags);
                     }
                     else