Move this to its own file (it's still a nested class though)

Terminal.Gui/Application.MainLoopSyncContext.cs

@@ -0,0 +1,51 @@
+namespace Terminal.Gui;
+
+public static partial class Application
+{
+    /// <summary>
+    ///     provides the sync context set while executing code in Terminal.Gui, to let
+    ///     users use async/await on their code
+    /// </summary>
+    private sealed class MainLoopSyncContext : SynchronizationContext
+    {
+        public override SynchronizationContext CreateCopy () { return new MainLoopSyncContext (); }
+
+        public override void Post (SendOrPostCallback d, object state)
+        {
+            MainLoop.AddIdle (
+                              () =>
+                              {
+                                  d (state);
+
+                                  return false;
+                              }
+                             );
+        }
+
+        //_mainLoop.Driver.Wakeup ();
+        public override void Send (SendOrPostCallback d, object state)
+        {
+            if (Thread.CurrentThread.ManagedThreadId == _mainThreadId)
+            {
+                d (state);
+            }
+            else
+            {
+                var wasExecuted = false;
+
+                Invoke (
+                        () =>
+                        {
+                            d (state);
+                            wasExecuted = true;
+                        }
+                       );
+
+                while (!wasExecuted)
+                {
+                    Thread.Sleep (15);
+                }
+            }
+        }
+    }
+}