AJAX focus does not work with multiple forms of the same type

Problem/Motivation

The Drupal AJAX re-focus logic does not work if the relevant form is instanced multiple times on the same page.

Only the first instance is automatically focused.

Steps to reproduce

On a page with duplicated AJAX forms, in which editing textfield A causes an AJAX callback which re-focuses textfield A afterwards:

1. edit textfield A in form 2
2. wait for AJAX callback
3. expect focus to be in textfield A of form 2, see that the actual focus is in textfield A of form 1

Proposed resolution

Adjust the re-focusing logic to respect the index of the original element. this might break if the AJAX callback causes the page to have a different number of form instances. But I suspect this case does not really happen.

Remaining tasks

• test
• implementation
• review

User interface changes

• improved accessibility