Could you clarify what you mean by packet? Are you talking about HTTP requests or about the new non-HTTP TCP streams? The UI/UX for the latter is very much work-in-progress.
The currently focused flow should never change when a new one comes in. If that's happening to you, please file a bug over on GitHub or ping me on our dev Slack and we'll get this fixed ASAP. :)
Do you have by any chance enabled focus to follow new requests? Keyboard shortcut Shift + F. Although it's disabled by default (IIRC), you may have enabled it by accident.