Use an Extension:
Install a third-party extension (such as uBlock Origin) that offers more robust pop-up/tab filtering. These can block unwanted new tabs even if they appear click-triggered.

@dan83m What about what's in https://forums.opera.com/topic/43601/instructions-on-posting-about-problems-in-opera-for-windows/1?

Version of Opera? Where the problem happens? When the problem happens? It would be a good start.

This is till a relevent topic, it's super annoying and makes me want to consider moving to another browser !!

I agree with the previous comments and suggestions. I work on two monitors and when switching from one to the other, an unwanted application from the sidebar is displayed all too often...
it always drives me crazy because I have to do the original operation again, but in order to avoid the sidebar...
it's very annoying
the whole problem would be solved by a simple option to set the display delay in milliseconds...
please please please!