How to reproduce

1. Launch Minecraft 1.12.2 or an earlier snapshot than the latest available.
2. Create a new single-player world from that version
3. Close the game
4. Launch Minecraft with the latest snapshot
5. Click 'Singleplayer'
6. Select the world you created previously in the earlier version/snapshot
7. Click Play

The "Do you really want to load this world?" screen confirms whether you want to upgrade this world or not.

Take note of the three options: "I know what I'm doing", "Backup and load" and "Cancel"

1. On the "Do you really want to load this world?" screen, Press the Esc key on your keyboard.

EXPECTED BEHAVIOUR: Pressing Esc should abort the request as if you pressed 'Cancel'.

OBSERVED BEHAVIOUR: The world loads up in the newer snapshot, converting itself without making a backup. It is as if the Esc key triggered "I know what I'm doing!"

RECOMMENDED FIX: Change the Esc key to become 'Cancel.' This should always be the default of someone hitting Esc.

Note: Other screens, such as the "Are you sure you want to delete this world?" screen behave the correct way - aborting the request rather than completing it.