Refactor SynchronizationContext to provide order and execution guarantees

osu.Framework.Tests/Visual/Drawables/TestSceneSynchronizationContext.cs

@@ -6,6 +6,7 @@
 using System.Threading.Tasks;
 using NUnit.Framework;
 using osu.Framework.Allocation;
+using osu.Framework.Development;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Shapes;
 using osu.Framework.Platform;
@@ -62,8 +63,12 @@ public void TestAsyncThrows()
                 // ReSharper disable once AsyncVoidLambda
                 host.UpdateThread.Scheduler.Add(async () =>
                 {
+                    Assert.That(ThreadSafety.IsUpdateThread);
+
                     await Task.Delay(100).ConfigureAwait(true);
 
+                    Assert.That(ThreadSafety.IsUpdateThread);
+
                     throw new InvalidOperationException();
                 });
             });

osu.Framework/Threading/SchedulerSynchronizationContext.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System.Diagnostics;
 using System.Threading;
 
 #nullable enable
@@ -19,6 +20,16 @@ public SchedulerSynchronizationContext(Scheduler scheduler)
             this.scheduler = scheduler;
         }
 
+        public override void Send(SendOrPostCallback d, object? state)
+        {
+            var del = scheduler.Add(() => d(state));
+
+            Debug.Assert(del != null);
+
+            while (del.State == ScheduledDelegate.RunState.Waiting)
+                scheduler.Update();
+        }
+
         public override void Post(SendOrPostCallback d, object? state) => scheduler.Add(() => d(state));
     }
 }