4.21.0

• Add TabsetCollapseButton and TabsetMaximizeButton components, which collapse (or expand) and maximize (or restore) the tabset they're rendered within, with aria-expanded/aria-controls and aria-pressed. They render nothing when the action isn't available (e.g. maximizing the only tabset). - #805
• The children, className, style, and title props of TabCloseButton, TabsetDragHandle, and the new buttons can be functions of the component's state, e.g. {({ isCollapsed }) => (isCollapsed ? "▸" : "▾")}. - #805
• Fix groupResizeBehavior: "preserve-pixel-size" panels drifting by a fraction of a pixel when panels have fractional sizes (e.g. at 150% display scaling): a Group's size was measured as the sum of its panels' rounded sizes, which could be off by a pixel and looked like the Group being resized. - #804