Previewing is a term Meural uses for sending an image to the Canvas to be displayed that is not officially on that device yet; it’s normally used to preview art you might want to buy from their art collection on the large screen.
Ah, this seems to be different for us. It is not permanent for me - it displays exactly as long as the preview duration you can set through the Meural web interface - that’s why I’m pretty sure postcard is the same as Meural’s preview system. I had that set to 60 seconds so after a minute it went back to my original playlist.
Server says: Apache/2.4.18 (Ubuntu)
Meural app says: 2.1.9
Device JSON says: V2.1.9_2.0.10
Which version Canvas do you have?