Change Proposal: Snackbar notification system

[zoharma]

What is being proposed?

Introduce a snackbar notification system for stacking and queueing multiple notifications (e.g. notistack or an equivalent MUI-compatible library), plus written guidance on when and how to use it.

(was raised by several issues in i15-1/Atlas project)

Why is this needed?

Today the DS only themes MUI's bare Snackbar/Alert (see MuiSnackbar/MuiSnackbarContent/MuiAlert overrides in DiamondDSTheme.ts and the Snackbar story). Each consumer wires its own open state and there's no way to stack or queue multiple notifications, and no guidance on when a snackbar is appropriate versus an inline Alert.

MUI/Material's own guidance recommends showing only one snackbar at a time, but our platform needs to support multiple concurrent, stacked notifications, as seen in systems such as IBM Carbon's notification component.

What will change?

• Behaviour: adds a SnackbarProvider (or similar) enabling stacking/queueing of multiple notifications
• Component props or usage: consumers trigger notifications imperatively instead of managing open state themselves
• New dependency: notistack (or equivalent), themed to match existing MuiAlert/MuiSnackbarContent styling
• Documentation: usage guidance covering when to use a snackbar vs. inline Alert, severity/variant mapping, duration and placement conventions

Interface changes (if any)

const { enqueueSnackbar } = useSnackbar();
enqueueSnackbar("Saved", { variant: "success" });

Breaking change?

• No

Next steps

A maintainer will review this issue.
If accepted, it will be marked as accepted and a PR may then be opened.