- Launch Minecraft 1.12.2 or an earlier snapshot than the latest available.
- Create a new single-player world from that version
- Close the game
- Launch Minecraft with the latest snapshot
- Click 'Singleplayer'
- Select the world you created previously in the earlier version/snapshot
- 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"
- 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.