Problem persisted. However, I get the feeling the problem is caused or influenced by VirtualBox guest additions. If guest additions are not used all is fine, if they are used opera resizing plays up.
You are right, it happens that Opera is stuck to full window mode when VBox Guest Additions are installed and all its clients started.
And it doesn't seem to be related to any hot key feature.
But it's hard to reproduce. The error occurs sometimes only, may depend on loading sequence ore alike.
I got the feeling too, it has something to do with size management of VBox Guest Additions in combination of Opera full window mode.
So maybe a workaround could be, first to get rid of Opera full window mode. It may be worth a try not to start VBox Guest clients or even uninstall VBox before installing Opera, then resizing Opera, to get rid of its full window mode and only afterwards add (reboot with) VBox Guest Additions and its clients.
And if it should be stuck again, close Opera, resize Virtual Box and start Opera again. This way I got rid of the error several times.
Good luck