To be more precise, Java initially uses a linked list for nodes within a bin. If the number of items inside the bin crosses TREEIFY_THRESHOLD (which is 8), then that specific bin is converted into a RB tree.
This hack has terrible accessibility. For one, the <label> is not focusable by default, so keyboard users won't be able to toggle the popup at all. And I don't think it's possible to dynamically set ARIA attributes without JS.