show_dropdown aria-expanded applied to wrong element

Currently `show_dropdown()` applies `aria-expanded` to the toggled dropdown element instead of the button toggling the dropdown.

> There are several widgets that can be expanded and collapsed, including menus, dialogs, and accordion panels. Each of these objects, in turn, has an interactive element that controls their opening and closing. The aria-expanded attribute is applied to this focusable, interactive control that toggles the visibility of the object.

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-expanded

Start by locating the show_dropdown() entry point and inspect which element currently receives aria-expanded. Compare that behavior with the linked ARIA guidance; done means the focusable button or other interactive control that toggles the dropdown receives the attribute, rather than the dropdown itself.

Written by the indexing model from the issue text.