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How to disable auto-save:
Go to File > Settings (Ctrl+Alt+S).
Go to Appearance & Behavior > System Settings.
Make sure the two are unchecked:
Save files on frame deactivation
Save files automatically if application is idle for x sec.
Go to Editor > General > Editor Tabs
Put a checkmark on "Mark modified files with asterisk"
(Optional but recommended) Under "Tab Closing Policy", select "Close non-modified files first". You may also want to increase the number of allowed tabs.
Click Apply > OK.