Format completion in VSCode with built-in command

Question

I'm creating the VS Code Language Server and Client. I want to be able to format the completion result before it is shown to the user and use the VS Code formatting capabilities.

When the completion is accepted on the Client side I can format it by creating a selection in the active editor and running formatSelection command on it programatically. Thus VSCode automatically detects and runs the formatter - default or configured by the user.

async function prettifyCompletion(completionStartPosition: vscode.Position, completionEndPosition: vscode.Position) {
  const editor = vscode.window.activeTextEditor;

  if (editor) {
    const newSelection = new vscode.Selection(completionStartPosition, completionEndPosition);

    editor.selection = newSelection;
    await vscode.commands.executeCommand('editor.action.formatSelection');
  }
}

The thing I'm trying to achieve is to run the same command before the completion is actually accepted by the user. So that when it is displayed in ghost text, it is already formatted. For that I try to get the result from the server before it is returned to the client in provideInlineCompletionItems middleware

 async provideInlineCompletionItems(
    document: vscode.TextDocument,
    position: vscode.Position,
    context: vscode.InlineCompletionContext,
    token: vscode.CancellationToken,
    next: ProvideInlineCompletionItemsSignature,
  ) {
 
      const result = await next(document, position, context, token);
      
      let prettifiedResult;
      
      // TODO: prettify result here
      
      return prettifiedResult;

}

The problem is that as the completion result it not rendered in the editor I can't create a Selection out of it to run a command on. I tried using the vscode.executeFormatRangeProvider built-in command. But it seems to produce no results.

UPDATE:

Another approach I've tried is creating a separate document where the formatting happens.

export async function prettifyInVirtualDocument(document: vscode.TextDocument, startPosition: vscode.Position) {
  const doc = await vscode.workspace.openTextDocument({content: document.getText(), language: document.languageId});
  const editor = await vscode.window.showTextDocument(doc);

  const endPosition = new vscode.Position(editor.document.lineCount - 1, editor.document.lineAt(editor.document.lineCount - 1).text.length);

  // Select the inserted text
  const selection = new vscode.Selection(startPosition, endPosition);
  editor.selection = selection;

  await vscode.commands.executeCommand('editor.action.formatSelection')
  const selectedText = editor.document.getText(editor.selection);
  return selectedText;
}

The problem with that is that is not a "virtual" document and a user can see the document in a separate tab. If it could only be hidden from them