Dynamically created overlays (toasts, tooltips, dropdowns) need automatic z‑index values to appear above other content without hardcoding. If multiple overlays exist, the newest should be on top, and freed indices should be reused to avoid gaps.
ZIndexManager allocates increasing z‑index values (starting from a base) and reuses released ones. It provides
allocate() to get a new z‑index and release(zIndex) to free it. This demo creates coloured boxes with allocated
z‑indexes; closing a box releases its index, which will be used for the next created box.
ToastContainer, Portal, StackedOverlays, FloatingElements, NotificationCenter