feat(GlCollapsibleListbox): fix keyboard navigation when selected is `null`
What does this MR do?
When the selected value was null in the single-select listbox, the focus would stay on the toggle. Now it moves to the first item or search input (if present)
How to test
- Run storybook locally
- Change src/components/base/new_dropdowns/listbox/listbox.stories.js#L144 to
null. Make sure that focus moves to the first item on listbox open.
Closes #2144 (closed)