I had the issue of a maximized Opera "fighting" not do be un-maximized or resized using latest LinuxMint (Cinnamon) –
close window decorations were visible, though. Using a swift mouse move it was possible to move the maximized Opera from one Screen to an adjacent, different sized monitor where it instantly maximized itself again. All those mouse interactions leading to flickering, by the way.
I checked any of Cinnamon's "snap to *" settings but what did get me Opera restored was to save the session with V7 Session sidebar extension, closing Opera, deleting "
Current Session" from the profile, and ultimately restoring the session from V7 Sessions.
I guess, there is a bug – and perhaps it is the same the OP was suffering from.