Noticed the same issue with latest update to Opera v55.0.2994.37 on Linux Mint 19 Cinnamon (all updates installed). Was working fine in all previous Opera versions - ie. could click outside Whatsapp or on the Whatsapp icon to close the sidebar.
Latest posts made by sleepyeyes
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Opera for LinuxRE: Whatsapp plugin won´t close.
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Future releasesRE: [Linux Bug] WhatsApp on new sidebar: doesn't hide when I press the icon again
Noticed the same issue with latest update to Opera v55.0.2994.37 on Linux Mint 19 Cinnamon (all updates installed). Was working fine in all previous Opera versions - ie. could click outside Whatsapp or on the Whatsapp icon to close the sidebar.
However, contrary to the OP, I find that I can still close it by pressing Ctrl+Shift+M.