Anchor positions
Six anchor positions, each with its own independent stack. Toasts queued to different anchors don't compete for the same maxSnack slot.
Provider default vs per-toast override
The provider's anchorOrigin sets the default for new toasts. A per-call anchorOrigin in enqueueSnackbar options wins.
Stack ordering
New toasts always appear at the front of their anchor's stack. Older toasts shift back, scale down, and fade. With maxSnack set, anchors that exceed the limit evict the oldest visible toast and fire onClose(_, 'maxsnack', key) on the evicted item.