When you have overlays turned on in editor, as soon as you edit the model it turns them off, it should stay on when modelling.
This is intended behaviour, please see our user manual, https://documentation.signavio.com/
This idea hasn't been implemented, and has been forwarded to our Product Managers as a Feature Request.
If you like this idea, please register your interest by creating a ticket in the SAP Signavio Support portal, referencing this knowledge base article.
We'd also like your business case, so we can better understand the needs of our customers.
KBA , BPI-SIG-PM-MOD-EDT , Graphical Editor for SAP Signavio Process Manager , Product Enhancement