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Save the cover to disc, then in Metadata editor, browse for cover. Really if it's anything important you might want a separate save of a cover anyway.
PS. I never use flatpack if a non-flatpack version is available. Flatpack is flaky & inferior.
I use Linux Mint 20.1, Mate Desktop. I have Chromium, Firefox and Waterfox browsers.
Viber, Calibre & Signal from their sites, not distros.
Copy & paste from Gimp to Viber makes Gimp exit. Copy & paste from Image Viewer to Viber does nothing.
I only put covers into Calibre via Download or Browse in metadata editor . I'd assumed the Calibre preview thumbnail of cover copy/paste is only for inside Calibre (which works).
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