The Cart pane does not properly attach its CSS to the checkout form

The continue button isn't actually triggered via AJAX, however if you do something like change a country, which does have an AJAX onchange event attached to it, then the form will validate as it's rebuilt. Can you post a screenshot of the form as you see it? And can we rule out just an incompatibility with the theme on your site?

I had the same problem. So I added the validation handler to the cart contents pane (as I need to validate the cart contents again after form submission):

/**
 * Implements hook_commerce_checkout_pane_info_alter().
 */
function mymodule_commerce_checkout_pane_info_alter(&$checkout_panes) {
  if (isset($checkout_panes['cart_contents'])) {
    // We just need to add our validation. However, the system requires to
    // add also the default callbacks:
    $checkout_panes['cart_contents']['callbacks'] = array(
      'checkout_form_validate' => 'mymodule_pane_cart_contents_validate',
      'settings_form' => 'commerce_cart_contents_pane_settings_form',
      'checkout_form' => 'commerce_cart_contents_pane_checkout_form',
      'checkout_form_submit' => 'commerce_cart_contents_pane_checkout_form_submit',
      'review' => 'commerce_cart_contents_pane_review',
    );
  }
}

Why didn't you just add a validate handler to the Continue button?

I can't remember details as I had this problem a few weeks ago.
However, adding the validation handler to the Continue button was of course the first thing I did. As I took a look at the default validation handler I saw that it just calls the panes validation handlers. So I used this and all problems were gone.

Adding the validation handler to the Continue button resulted in broken styles if validation failed (see screenshot of #2).

I am also having the same issue as #1 when using Kickstart 2.

Adding a custom validation handler to the continue button does not load the correct CSS files when changing the country field.

Attaching the CSS files manually did not work for me and the hook_commerce_checkout_pane_info_alter code in #3 loads the CSS OK but the checkout form does not advance to the next pane (no message why).

Ok, I was able to reproduce this using the code in the original post. What was happening was the checkout pane was simply using drupal_add_css() instead of properly attaching its CSS to its form element in the first place. Thus when the form was rebuilt on a validation error, the CSS wasn't being attached (as expected).

It's an easy enough fix to just change the way we attach it.

Unfortunately, I can't use #attached in the display formatter output for the price field, so we will just have to make an accommodation and attach it to the pane as well. I don't really like that, because it means we open the door to attach any ol' CSS file to the form (and I'm not even sure how that plays with RTL support). But I don't see a way around that.

Commit: http://drupalcode.org/project/commerce.git/commitdiff/8122dd1