[Blazor] JS root component registration (#37180)

* Change to trigger callbacks only after the renderer has been attached
* Update Blazor JS files

Author: javiercn

src/Components/Web.JS/src/JSInitializers/JSInitializers.ts

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 import { Blazor } from '../GlobalExports';
+import { rendererAttached } from "../Rendering/WebRendererInteropMethods";
 
 type BeforeBlazorStartedCallback = (...args: unknown[]) => Promise<void>;
 export type AfterBlazorStartedCallback = (blazor: typeof Blazor) => Promise<void>;
@@ -38,6 +39,7 @@ export class JSInitializer {
   }
 
   async invokeAfterStartedCallbacks(blazor: typeof Blazor): Promise<void> {
+    await rendererAttached;
     await Promise.all(this.afterStartedCallbacks.map(callback => callback(blazor)));
   }
 }

src/Components/Web.JS/src/Rendering/WebRendererInteropMethods.ts

@@ -7,6 +7,12 @@ import { enableJSRootComponents, JSComponentParametersByIdentifier, JSComponentI
 
 const interopMethodsByRenderer = new Map<number, DotNet.DotNetObject>();
 
+let resolveRendererAttached : () => void;
+
+export const rendererAttached = new Promise<void>((resolve) => {
+  resolveRendererAttached = resolve;
+});
+
 export function attachWebRendererInterop(
   rendererId: number,
   interopMethods: DotNet.DotNetObject,
@@ -23,6 +29,8 @@ export function attachWebRendererInterop(
     const manager = getInteropMethods(rendererId);
     enableJSRootComponents(manager, jsComponentParameters, jsComponentInitializers);
   }
+
+  resolveRendererAttached();
 }
 
 export function dispatchEvent(browserRendererId: number, eventDescriptor: EventDescriptor, eventArgs: any): void {