@sgunhouse: I do have an F11 key on my Mac keyboard, but it mainly functions as the "turn down the volume" key. Somehow I must have accidentally triggered the F11 function while lowering the volume, thus the frustration. I'll know for the next time it happens. Thanks for the info.

Thanks. I found the option in Safari under the View menu, and lo and behold, it was also in Opera. Such a simple fix—once you know it. So I'm back to normal now. I appreciate the help.

Opera on my 2020 iMac suddenly and annoyingly went into fullscreen mode, hiding the menu bar and the dock. It creates an up to 10 or 15 second pause when I try to go to another app. I want to be able to see the menu and dock at all times. How do I get back to non-fullscreen mode??