Using `vscode.RelativePattern` and `vscode.workspace.findFiles`

I've changed the helloworld-web-sample with this simple testing code below but it can't find files when running it with the vscode-test-web (files.length always 0). It works fine when I run it using the Run Web Extension in VS Code though (files.length > 0).

Is there a different way of using vscode.RelativePattern and vscode.workspace.findFiles for browser? Thanks!

diff --git a/helloworld-web-sample/src/web/extension.ts b/helloworld-web-sample/src/web/extension.ts
index 176a2dd..482892a 100644
--- a/helloworld-web-sample/src/web/extension.ts
+++ b/helloworld-web-sample/src/web/extension.ts
@@ -13,11 +13,13 @@ export function activate(context: vscode.ExtensionContext) {
 	// The command has been defined in the package.json file
 	// Now provide the implementation of the command with registerCommand
 	// The commandId parameter must match the command field in package.json
-	let disposable = vscode.commands.registerCommand('helloworld-web-sample.helloWorld', () => {
+	let disposable = vscode.commands.registerCommand('helloworld-web-sample.helloWorld', async () => {
 		// The code you place here will be executed every time your command is executed
 
 		// Display a message box to the user
-		vscode.window.showInformationMessage('Hello World from helloworld-web-sample in a web extension host!');
+		const relativePattern = new vscode.RelativePattern(vscode.workspace.workspaceFolders![0], "**/*");
+		const files = await vscode.workspace.findFiles(relativePattern);
+		await vscode.window.showInformationMessage(`Hello World from helloworld-web-sample in a web extension host! ${files.length}`);
 	});
 
 	context.subscriptions.push(disposable);

Start with helloworld-web-sample/src/web/extension.ts and reproduce the issue through vscode-test-web using the shown RelativePattern and findFiles call. Compare the browser run with Run Web Extension in VS Code and determine the expected browser behavior; done means the sample's file lookup works as intended or the limitation and required usage are documented.