fix: freezes encountered by some users (#508)

Previously, `TaskUtil.OnMainThread` was asynchronous, allowing for the
editor to remain responsive even if something was spamming
invalidations. However, with the prior change fdaee91, this was
accidentally changed to be synchronous. This seems to cause editor
freezes in some situations - particularly, if OnMainThread was invoked
early in editor initialization, before we know which thread is the main
thread.

This change reverts this logic to be asynchronous.

NOTE: This also reverts the accidental addition of a public API in 1.6.3

Closes: #507

CHANGELOG.md

@@ -10,10 +10,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 ### Fixed
+- [#507] Fixed a potential editor freeze bug
 
 ### Changed
 
 ### Removed
+- [#507] Removed the `NDMFSyncContext.RunOnMainThread` API which was accidentally addded in 1.6.3.
+  This might be re-added in 1.7.0.
 
 ### Security
 

Editor/PreviewSystem/ComputeContext.cs

@@ -145,7 +145,7 @@ public ComputeContext(string description)
                     arg0: this
                 ).EventId;
 
-                TaskUtil.OnMainThread(this, DoInvalidate);
+                NDMFSyncContext.RunOnMainThread(target => DoInvalidate((ComputeContext)target), this);
             };
             Description = description;
 

Editor/PreviewSystem/Task/NDMFSyncContext.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Threading;
+using JetBrains.Annotations;
 using UnityEditor;
 using UnityEngine;
 
@@ -15,20 +16,23 @@ public static class NDMFSyncContext
     {
         public static SynchronizationContext Context = new Impl();
         private static Impl InternalContext = (Impl)Context;
-
+        
         /// <summary>
-        /// Runs the given function on the unity main thread - synchronously if possible.
+        ///    Runs the given function on the unity main thread, passing the given target as an argument.
+        ///    The function will be invoked synchronously if called from the main thread; otherwise, it will be
+        ///    run asynchronously.
         /// </summary>
-        /// <param name="cb"></param>
-        public static void RunOnMainThread(EditorApplication.CallbackFunction cb)
+        /// <param name="receiver"></param>
+        /// <param name="target"></param>
+        internal static void RunOnMainThread(SendOrPostCallback receiver, object target)
         {
             if (Thread.CurrentThread.ManagedThreadId == InternalContext.unityMainThreadId)
             {
-                cb();
+                receiver(target);
             }
             else
             {
-                Context.Send(_ => cb(), null);
+                Context.Post(receiver, target);
             }
         }
 

Editor/PreviewSystem/TaskUtil.cs

@@ -13,10 +13,5 @@ static void Init()
         {
             _mainThread = Thread.CurrentThread;
         }
-
-        internal static void OnMainThread<T>(T target, Action<T> receiver)
-        {
-            NDMFSyncContext.RunOnMainThread(() => receiver(target));
-        }
     }
 }