Multiple overlays open at the same time (dialog on top of another dialog) need proper z‑index stacking and focus management. The newer overlay should appear above older ones, and closing it should restore focus.
StackManager manages an overlay stack (push, pop, remove). It assigns increasing z‑index values, and can manage focus between
stacked overlays (focus restoration handled by the caller). This demo creates floating panels that stack.
Dialog, Modal, Popover, Tooltip, Toast, ContextMenu