Ctools Modal form's #id changes when validation fails

This is probably happening because Drupal records what HTML IDs are on the page and form api guarantees form IDs are unique. When I'm doing stuff in the modal, I often unset...I think it's $_POST['ajax_html_ids'] to ensure this doesn't happen, since modals are pretty insulated from everything else anyway.

This is probably going to be a bigger and bigger problem moving forward. I think this probably just needs to be documented; we shouldn't do it automatically but document that people may need to do it.

I guess this is the drupal function that's called to give unique ids drupal_html_id($id) and looking at that function you're correct with $_POST['ajax_html_ids']. You can see a quote at the bottom from Ryan Szrama warning about the same behaviour.

But this still doesn't make sense as to why it doesn't rename the id for the user_register_form. To add to Robert's issue summary, the renaming of the id also happens with user_login........what makes user_register_form so special?

I just hit this again in #1675522: Remove button broken if form is rebuilt (via ajax or failed validation).

field_collection has this same pattern (already in CTools and Views) as written by @merlinofchaos, and it solves my problem, and is unlikely to cause additional adverse side-effects.

Per @dawehner's request, making this opt-in.

Reviewed and looks good, per Tim. Committed!