Problem/Motivation
jquery ui-autocomplete dropdown is hidden by ui-dialog popup, but should be z-indexed above
Steps to reproduce
1. You have to create the link which is triggering ui-dialog by click.
2. Put the form inside dialog's content
3. Add at least one field with autocomplete into this form.
4. Try to open dialog and type something into autocomplete and you will not see ui-autocomplete, since it's located "under" ui-dialog.
If everything is ok -> try to put ui-dialog not in the root of document, but with appendTo option defined https://api.jqueryui.com/dialog/#option-appendTo.
Proposed resolution
1. Decide and create unique z-index for ui-autocomplete, which should have higher value than ui-dialog and update this page https://www.drupal.org/docs/theming-drupal/z-indexes-in-drupal-8
2. Update all related core themes.
Current behavior
Expected behavior
