WIP

src/Browser/Avalonia.Browser/BrowserAppBuilder.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using Avalonia.Browser.Interop;
+using Avalonia.Browser.Rendering;
 using Avalonia.Metadata;
 
 namespace Avalonia.Browser;
@@ -109,6 +110,10 @@ public static class BrowserAppBuilder
         AvaloniaLocator.CurrentMutable.Bind<BrowserPlatformOptions>().ToConstant(options);
 
         await AvaloniaModule.ImportMain();
+        if (BrowserWindowingPlatform.IsThreadingEnabled)
+        {
+            await RenderWorker.InitializeAsync();
+        }
 
         if (builder.WindowingSubsystemInitializer is null)
         {

src/Browser/Avalonia.Browser/Interop/AvaloniaModule.cs

@@ -6,11 +6,13 @@ namespace Avalonia.Browser.Interop;
 
 internal static partial class AvaloniaModule
 {
-    private static readonly Lazy<Task> s_importMain = new(() =>
+    private static readonly Lazy<Task> s_importMain = new(ImportMainToCurrentContext);
+    
+    public static Task ImportMainToCurrentContext()
     {
         var options = AvaloniaLocator.Current.GetService<BrowserPlatformOptions>() ?? new BrowserPlatformOptions();
         return JSHost.ImportAsync(MainModuleName, options.FrameworkAssetPathResolver!("avalonia.js"));
-    });
+    }
 
     private static readonly Lazy<Task> s_importStorage = new(() =>
     {

src/Browser/Avalonia.Browser/Rendering/BrowserRenderTimer.cs

@@ -22,16 +22,8 @@ internal class BrowserRenderTimer : IRenderTimer
     {
         add
         {
-            if (!_started)
+            if (!BrowserWindowingPlatform.IsThreadingEnabled)
-            {
+                StartOnThisThread();
-                _started = true;
-                if (BrowserWindowingPlatform.IsThreadingEnabled)
-                {
-                    
-                }
-                else
-                    TimerHelper.RunAnimationFrames(RenderFrameCallback);
-            }
 
             _tick += value;
         }
@@ -41,6 +33,15 @@ internal class BrowserRenderTimer : IRenderTimer
         }
     }
 
+    public void StartOnThisThread()
+    {
+        if (!_started)
+        {
+            _started = true;
+            TimerHelper.RunAnimationFrames(RenderFrameCallback);
+        }
+    }
+
     private bool RenderFrameCallback(double timestamp)
     {
         if (_tick is { } tick)

src/Browser/Avalonia.Browser/Rendering/RenderWorker.cs

@@ -1,6 +1,81 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Runtime.InteropServices.JavaScript;
+using System.Threading.Tasks;
+using Avalonia.Browser.Interop;
+
 namespace Avalonia.Browser.Rendering;
 
-internal class RenderWorker
+internal partial class RenderWorker
 {
+    [DllImport("*")]
+    private static extern int pthread_self();
+
+    [JSImport("WebRenderTargetRegistry.initializeWorker", AvaloniaModule.MainModuleName)]
+    private static partial void InitializeRenderTargets(); 
+    
+    public static int WorkerThreadId;
     
+    public static Task InitializeAsync()
+    {
+        var tcs = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var workerTask = JSWebWorkerWrapper.RunAsync(async () =>
+        {
+            try
+            {
+                await AvaloniaModule.ImportMainToCurrentContext();
+                InitializeRenderTargets();
+                WorkerThreadId = pthread_self();
+                BrowserCompositor.RenderTimer.StartOnThisThread();
+                tcs.SetResult();
+                // Never surrender
+                await new TaskCompletionSource().Task;
+            }
+            catch (Exception e)
+            {
+                tcs.SetException(e);
+            }
+        });
+        
+        workerTask.ContinueWith(_ =>
+        {
+            if (workerTask.IsFaulted)
+                tcs.TrySetException(workerTask.Exception);
+        });
+        return tcs.Task;
+    }
+    
+    class JSWebWorkerWrapper
+    {
+
+        [DynamicDependency(DynamicallyAccessedMemberTypes.PublicMethods, "System.Runtime.InteropServices.JavaScript.JSWebWorker", 
+            "System.Runtime.InteropServices.JavaScript")]
+        [UnconditionalSuppressMessage("Trimming", 
+            "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access otherwise can break functionality when trimming application code",
+            Justification = "Private runtime API")]
+        static JSWebWorkerWrapper()
+        {
+            var type = typeof(System.Runtime.InteropServices.JavaScript.JSHost)
+                .Assembly!.GetType("System.Runtime.InteropServices.JavaScript.JSWebWorker");
+#pragma warning disable IL2075
+            var m = type!
+
+                .GetMethods(BindingFlags.Static | BindingFlags.Public
+                ).First(m => m.Name == "RunAsync"
+                             && m.ReturnType == typeof(Task)
+                             && m.GetParameters() is { } parameters
+                             && parameters.Length == 1
+                             && parameters[0].ParameterType == typeof(Func<Task>));
+
+#pragma warning restore IL2075
+            RunAsync = (Func<Func<Task>, Task>) Delegate.CreateDelegate(typeof(Func<Func<Task>, Task>), m);
+
+        }
+
+        public static Func<Func<Task>, Task> RunAsync { get; set; }
+    }
+
 }

src/Browser/Avalonia.Browser/WindowingPlatform.cs

@@ -65,7 +65,6 @@ internal class BrowserWindowingPlatform : IWindowingPlatform
             .Bind<IKeyboardDevice>().ToConstant(s_keyboard)
             .Bind<IPlatformSettings>().ToSingleton<BrowserPlatformSettings>()
             .Bind<IDispatcherImpl>().ToSingleton<BrowserDispatcherImpl>()
-            .Bind<IRenderTimer>().ToConstant(ManualTriggerRenderTimer.Instance)
             .Bind<IWindowingPlatform>().ToConstant(instance)
             .Bind<IPlatformGraphics>().ToConstant(new BrowserSkiaGraphics())
             .Bind<IPlatformIconLoader>().ToSingleton<IconLoaderStub>()

src/Browser/Avalonia.Browser/webapp/modules/avalonia/timer.ts

@@ -3,10 +3,10 @@ export class TimerHelper {
         function render(time: number) {
             const next = renderFrameCallback(time);
             if (next) {
-                window.requestAnimationFrame(render);
+                self.requestAnimationFrame(render);
             }
         }
 
-        window.requestAnimationFrame(render);
+        self.requestAnimationFrame(render);
     }
 }