eminuxCRM/app/javascript/controllers/test_mail_controller.js

import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = ["frame"]
  static values = { previewUrl: String }

  connect() {
    this.loadPreview()
  }

  preview() {
    clearTimeout(this.timer)
    this.timer = setTimeout(() => this.loadPreview(), 350)
  }

  loadPreview() {
    if (!this.hasFrameTarget) return

    const params = new URLSearchParams(new FormData(this.element))
    params.delete("authenticity_token")
    params.delete("commit")
    this.frameTarget.src = `${this.previewUrlValue}?${params.toString()}`
  }

  disconnect() {
    clearTimeout(this.timer)
  }
}