send.php

<?php

// process form
// respond as HTML with updated state

send_form-controller.js

import { Controller } from 'stimulus';

export default class extends Controller {
  submit(event) {
    this.inProgress(true);
    
    window.fetch('/send', {
      body: new FormData(event.target)
    }).then(response => {
      if (response.status !== 200) {
          throw new Error(`Failed!`);
      }
      return response.json();
    }).then(
      data => {
        this.setInProgress(false);
      },
      error => {
        this.setInProgress(false);
      }
    );
  }
  
  set inProgress(value) {
    this.data.set('inProgress', true);
  }
  
  get inProgress() {
    return JSON.parse(this.data.get('inProgress'));
  }
}

template.php

<form action="/send.php" data-controller="send_form" data-action="send_form#submit" data-in-progress="true">
  <input type="text" />
  <button>Send</button>
</form>