Problem

You're editing docker-compose.yaml in the side panel. Save accidentally inserts a tab character in front of ports:, breaking YAML indentation. The editor surface is supposed to block the save with a parser error before it reaches disk. Confirm the guard fires.

Acceptance

You've solved it when:

• Edit injects a tab-indent in front of ports: and Save is rejected

• The on-disk file is unchanged after the rejected save.
• You then fix the indentation and the same Save succeeds.