Collapsable/expandable panels in admin are not accessible

[bramd]

There are various accessibility issues with the collapsable/expandable panels in the admin. This panels can be found on the dashboard and in edit screens for posts/pages/etc:

• The collapse/expand is clickable, but has no semantic meaning
• The clickable element to expand/collapse a panel is not keyboard focusable

Suggested fixes:

• Use an a or button element, or apply an appropriate aria role (however, using of a native element is preferable here)
• If using an ARIA role, ensure to add tabindex to make it focusable
• Use the aria-expanded attribute to present the collapsed/expanded status of the panel

[joedolson]

Fixed in r33762