Read from sessionStorage

We need the checkout flow to persist choices for this browser session only. On page load, read cartTemp from sessionStorage; if present, hydrate the cart state and mark the UI as ready. If not present, start with an empty cart. After any item add/remove or quantity change, write the updated cartTemp JSON back into sessionStorage.

Before I hand this to QA, make the session-cart obvious and auditable. Show a small Session cart indicator with the count and lastUpdated time. Make pricing changes reflect immediately and add a Clear session cart button that deletes cartTemp. Flag any edge cases where sessionStorage may be cleared unexpectedly (private mode, different tabs) so the reviewer understands limits.

I tested the checkout across two tabs. I emptied the cart in tab A to complete purchase; tab B still showed items and the user tried to submit. On submit the app read sessionStorage and failed validation. I can't tell if tab B read stale sessionStorage or if writes weren't synchronized. What's the most likely diagnosis and the fastest fix to keep the UI consistent across tabs for this session-scoped cart?

Over time we keep getting tickets where users expect their cart to persist across tabs or after a reload but sessionStorage clears. Our shortcuts to 'fix' it introduce race conditions. What's one change to our development habit and one small runtime pattern to reduce these session-store surprises and improve predictability for users?