Not sure what you mean by "an unfilled rectangle."
Drawn in copper track?
Silkscreen?
As a Solid Region?
Which Gerber viewer?
Can you post the URL of an example footprint that demonstrates this?
Thanks.
Use gerbv as your Gerber viewer:
[https://docs.easyeda.com/en/Export/Export-Gerber-Files/index.html#Gerber-View](https://docs.easyeda.com/en/Export/Export-Gerber-Files/index.html#Gerber-View)
If they're ok in gerbv then they should be good for JLCPCB.
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