Adds an option that enables the rendering runs instantly.

Before this commit: UI -> render -> UI -> render （then it rendered)
After this commit: UI -> render (then it rendered)
Yes, It reduced rendering delay at least one frame.

src/Avalonia.Base/Media/MediaContext.Compositor.cs

@@ -21,9 +21,16 @@ private CompositionBatch CommitCompositor(Compositor compositor)
         var commit = compositor.Commit();
         _requestedCommits.Remove(compositor);
         _pendingCompositionBatches[compositor] = commit;
-        commit.Processed.ContinueWith(_ =>
-            _dispatcher.Post(() => CompositionBatchFinished(compositor, commit), DispatcherPriority.Send),
-            TaskContinuationOptions.ExecuteSynchronously);
+        if (_dispatcherOptions.InstantRendering)
+        {
+            CompositionBatchFinished(compositor, commit);
+        }
+        else
+        {
+            commit.Processed.ContinueWith(_ =>
+                    _dispatcher.Post(() => CompositionBatchFinished(compositor, commit), DispatcherPriority.Send),
+                TaskContinuationOptions.ExecuteSynchronously);
+        }
         return commit;
     }
 

src/Avalonia.Base/Media/MediaContext.cs

@@ -26,6 +26,7 @@ private record  TopLevelInfo(Compositor Compositor, CompositingRenderer Renderer
 
     private List<Action>? _invokeOnRenderCallbacks;
     private readonly Stack<List<Action>> _invokeOnRenderCallbackListPool = new();
+    private readonly DispatcherOptions _dispatcherOptions;
 
     private readonly DispatcherTimer _animationsTimer = new(DispatcherPriority.Render)
     {
@@ -37,13 +38,14 @@ private record  TopLevelInfo(Compositor Compositor, CompositingRenderer Renderer
 
     private readonly Dictionary<object, TopLevelInfo> _topLevels = new();
 
-    private MediaContext(Dispatcher dispatcher, TimeSpan inputStarvationTimeout)
+    private MediaContext(Dispatcher dispatcher, DispatcherOptions dispatcherOptions)
     {
         _render = Render;
         _inputMarkerHandler = InputMarkerHandler;
         _clock = new(this);
         _dispatcher = dispatcher;
-        MaxSecondsWithoutInput = inputStarvationTimeout.TotalSeconds;
+        _dispatcherOptions = dispatcherOptions;
+        MaxSecondsWithoutInput = dispatcherOptions.InputStarvationTimeout.TotalSeconds;
         _animationsTimer.Tick += (_, _) =>
         {
             _animationsTimer.Stop();
@@ -62,7 +64,7 @@ public static MediaContext Instance
             if (context == null)
             {
                 var opts = AvaloniaLocator.Current.GetService<DispatcherOptions>() ?? new();
-                context = new MediaContext(Dispatcher.UIThread, opts.InputStarvationTimeout);
+                context = new MediaContext(Dispatcher.UIThread, opts);
                 AvaloniaLocator.CurrentMutable.Bind<MediaContext>().ToConstant(context);
             }
 

src/Avalonia.Base/Rendering/Composition/Compositor.cs

@@ -38,6 +38,7 @@ public partial class Compositor
         private readonly List<Action> _pendingServerCompositorPostTargetJobs = new();
         private DiagnosticTextRenderer? _diagnosticTextRenderer;
         private readonly Action _triggerCommitRequested;
+        private readonly DispatcherOptions _dispatcherOptions;
 
         internal IEasing DefaultEasing { get; }
 
@@ -89,6 +90,7 @@ internal Compositor(IRenderLoop loop, IPlatformGraphics? gpu,
             _batchObjectPool = new(reclaimBuffersImmediately);
             _server = new ServerCompositor(loop, gpu, options, _batchObjectPool, _batchMemoryPool);
             _triggerCommitRequested = () => scheduler.CommitRequested(this);
+            _dispatcherOptions = AvaloniaLocator.Current.GetService<DispatcherOptions>() ?? new();
 
             DefaultEasing = new SplineEasing(new KeySpline(0.25, 0.1, 0.25, 1.0));
         }
@@ -111,7 +113,7 @@ public CompositionBatch RequestCompositionBatchCommitAsync()
                 using var _ = NonPumpingLockHelper.Use();
                 _nextCommit = new ();
                 var pending = _pendingBatch;
-                if (pending != null)
+                if (pending != null && !_dispatcherOptions.InstantRendering)
                     pending.Processed.ContinueWith(
                         _ => Dispatcher.Post(_triggerCommitRequested, DispatcherPriority.Send),
                         TaskContinuationOptions.ExecuteSynchronously);

src/Avalonia.Base/Threading/DispatcherOptions.cs

@@ -18,4 +18,10 @@ public class DispatcherOptions
     /// the same thread as rendering.
     /// </remarks>
     public TimeSpan InputStarvationTimeout { get; set; } = TimeSpan.FromSeconds(1);
+
+    /// <summary>
+    /// Gets or sets a value indicating whether rendering should be performed synchronously.<br/>
+    /// Otherwise, rendering will be invoked from UI thread to the render thread, back to the UI thread and then back to the render thread.
+    /// </summary>
+    public bool InstantRendering { get; set; } = false;
 }