Fixed some potential threading issues

src/Avalonia.Base/Threading/DispatcherFrame.cs

@@ -91,31 +91,44 @@ public class DispatcherFrame
 
     internal void Run(IControlledDispatcherImpl impl)
     {
-        // Since the actual platform run loop is controlled by a Cancellation token, we are restarting 
-        // it if frame still needs to run
-        while (Continue)
-            RunCore(impl);
-    }
-
-    private void RunCore(IControlledDispatcherImpl impl)
-    {
-        if (_isRunning)
-            throw new InvalidOperationException("This frame is already running");
-        _isRunning = true;
-        try
-        {
-            _cancellationTokenSource = new CancellationTokenSource();
-            // Wake up the dispatcher in case it has pending jobs
-            Dispatcher.RequestProcessing();
-            impl.RunLoop(_cancellationTokenSource.Token);
-        }
-        finally
+        Dispatcher.VerifyAccess();
+        
+        // Since the actual platform run loop is controlled by a Cancellation token, we have an
+        // outer loop that restarts the platform one in case Continue was set to true after being set to false
+        while (true)
         {
-            _isRunning = false;
-            _cancellationTokenSource?.Cancel();
-            _cancellationTokenSource = null;
+            // Take the instance lock since `Continue` is changed from one too
+            lock (Dispatcher.InstanceLock)
+            {
+                if (!Continue)
+                    return;
+                
+                if (_isRunning)
+                    throw new InvalidOperationException("This frame is already running");
+
+                _cancellationTokenSource = new CancellationTokenSource();
+                _isRunning = true;
+            }
+
+            try
+            {
+                // Wake up the dispatcher in case it has pending jobs
+                Dispatcher.RequestProcessing();
+                impl.RunLoop(_cancellationTokenSource.Token);
+            }
+            finally
+            {
+                lock (Dispatcher.InstanceLock)
+                {
+                    _isRunning = false;
+                    _cancellationTokenSource?.Cancel();
+                    _cancellationTokenSource?.Dispose();
+                    _cancellationTokenSource = null;
+                }
+            }
         }
     }
+    
 
     internal void MaybeExitOnDispatcherRequest()
     {

src/Avalonia.Controls/Platform/ManagedDispatcherImpl.cs

@@ -58,6 +58,10 @@ public class ManagedDispatcherImpl : IControlledDispatcherImpl
     
     public void RunLoop(CancellationToken token)
     {
+        CancellationTokenRegistration registration = default;
+        if (token.CanBeCanceled) 
+            registration = token.Register(() => _wakeup.Set());
+
         while (!token.IsCancellationRequested)
         {
             bool signaled;
@@ -105,5 +109,7 @@ public class ManagedDispatcherImpl : IControlledDispatcherImpl
             else
                 _wakeup.WaitOne();
         }
+
+        registration.Dispose();
     }
 }