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I can seems to be able to add new line character ("\n") to a properties file using the editor. I can make them manually by editing the file directly but not though the gui editor.
Confirmed. Thank you for the bug report.
I was able to reproduce it with platform PC+Linux, too.
Fixed - multiline values can be entered in the textarea at the bottom of the GUI editor. (Don't enter \n directly, instead write the value on multiple lines and the newlines will be converted to \n when the data are stored into the properties file).