Currently the collapse/expand button of the sidebar doesn't have a11y implementation, because in Confluence we use the Soy template function we can't add this implementation ourselves (it might be possible from the JS implementation) but we should probably add the implementation out of the box.
Usage described in documentation
Demonstration of all states
Interactive state (screen readers)
Visual regression tests
Navigation (only keyboard / only mouse)
Component render legibly
Element status (At Rest/Pressed/Selected/Read Only/Disabled)