How should I test interactive React UIs?

Updating the UI immediately when an action starts, then rolling back if the server request fails. This makes the UI feel instant instead of waiting for the server, which is great for actions like adding to a watchlist.

Use optimistic updates for instant feedback, debounce or throttle expensive inputs like search, keep re-renders in check by memoizing where necessary and keeping state local, and animate with purpose rather than for decoration.

Manage focus. When a modal opens, move focus into it. When it closes, return focus to the trigger element. Also handle escape to close and backdrop click, so keyboard and screen reader users can use the modal properly.