76b5726: SelectMenu: Added hierarchical menus. Set isHierarchical on the menu to turn it on and write every row as the new SelectMenuTreeItem, whose label is the row's content and whose children are the rows nested under it. Expansion is uncontrolled by default and controlled through defaultExpandedIds, expandedIds, and onExpandedChange.

SelectMenuItem takes a selectionState for menus where a parent's selection cascades onto its children, and the new useCascadingSelection hook derives those states from a tree.

Listbox takes a role, either listbox or tree.

61b1481: ButtonGroup: Adds a new component that joins related actions into one connected control. Use Button and IconButton directly as its children; the group sets their size, variant, className, and style, and an explicit prop on an item still wins. Popover and ActionBar reset these values, whether or not they render through a portal. DropdownMenuTrigger takes the group size and variant.