Hello @Jason Selby ,
There is not an option to alter the default line spacing in the editor, however to do a single line break the editor follows the industry standard for Soft Break (Shift+Enter) as a keyboard shortcut to allow dropping single spacing line breaks vs standard double line break to issue a new paragraph used when pressing enter alone for a Full Break.
If this is something you would like to see I would recomend providing this feed back on the following Feature requests that our Jira Service Desk Development teams put together to collect feedback on the new editor, one is for the externally visible customer portal and the other for the internal user view:
Hello there Cloud Community members! Today, we’re thrilled to announce Jira Service Management, the next generation of Jira Service Desk. Jira Service Management touts advancements in IT service ...
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