It must be achieved using CSS.
I have a tool that uses CKEditor, and I see the body iframe tag in which the content is being edited has a .cke_show_borders class.
So you can do:
.cke_show_borders { overflow: scroll; }
Or have more granular control over vertical / horizontal scrollbars
.cke_show_borders { overflow-y: scroll; // vertical scrollbar overflow-x: scroll; // horizontal scrollbar }
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