Opera is really a great browser with lots of feature. Even bookmark synchronization is great feature for ability to access bookmark, but when we access bookmark, user should have choice for viewability. It is a recommended feature request to opera team in year 2021.
@pedroncio By transparent you mean invisible. Usually means Opera thinks the window is off the screen, such as on a second monitor which was since unplugged. I've seen solutions such as Alt+spacebar to display the program's system menu then select Move to move it onto the screen, or half a dozen others.