Does me leaving a battlefield end Showdown?

No, leaving a battlefield does not end a showdown.

According to RiftJudge FAQ #7670, #3405, and #976, a showdown only ends when both players pass priority (Focus) consecutively without starting a new chain (playing a spell or activating an ability). It does not matter if one or both players no longer have units present at that battlefield.

Key points regarding this:

• Ongoing Showdown: If a showdown has already begun, it continues until both players pass focus in a row, even if all units are removed from the battlefield before damage calculation.
• Play Opportunities: Players may continue to play spells, actions, and reactions during the ongoing showdown, even if no units remain on the battlefield.
• Combat Resolution: If it is a combat showdown, it will proceed to damage calculation and combat cleanup regardless of whether units are still present.

If you have specific concerns about how the showdown state is managed on your battlefield, feel free to share the details!