The monitor is ~ 1 year old (EIZO FlexScan EV2455). It is connected to my laptop. I'm using the laptop with both screens (laptop and stand alone monitor) side by side. The strange behaviour is only at the main screen (stand alone monitor). If I move the Opera window to the other monitor Operas menu behaves normal if maximised. When the Opera window is not maximised the size and position of the menu is fine/normal, on both screens.
The upper part of the title bar of Opera - and only of this Program! - can not be seen if I maximise the window. The screen ends where the picture ends (see another picture here). The mouse can not disappear at the top. It stops where the picture ends at the top.
I noticed that the setting to hide the upper space between tabs and window/screen when maximised, does modify nothing on both monitors if I maximise Operas window.
For me it looks like Operas window recognises the size of the screen, but Operas menu does't recognize that the monitor scale in the MS-Windows settings was set to 125%, so the menu "thinks" in 100% monitor scale and therefore the size of the menu is too big.
I noticed a similar misbehave with the bookmarks "favicons". See this picture. The size of Operas icon window is larger than the icon, so that the edge of the next icon can be seen at the right and bottom of this mini-window.