discourse/app/views/users/activate_account.html.erb

<div id='simple-container'>

    <h2><%= t 'activation.welcome_to', site_name: SiteSetting.title %></h2>
    <br/>
    <button class='btn' id='activate-account-button'><%= t 'activation.action' %></button>

    <%= form_tag(perform_activate_account_path, method: :put, id: 'activate-account-form') do %>
    <% end %>
</div>

<script language="javascript">
  (function() {
   var t1 = new Date().getTime(),
       button = document.getElementById('activate-account-button'),
       form = document.getElementById('activate-account-form');

    button.addEventListener('click', function() {
      var diff = new Date().getTime() - t1;

      // Ensure the form has been visible for a few ms before allowing the
      // user to submit.
      if (diff > 50) {
        form.submit();
      }
    });
  })();
</script>