@moriz I have some steps to you to do.
1° - is only happens in Opera Browser?
2° - you can send a PrintScreen when this occurs ?
Note: You can upload your image on imgur and send the link to us to help you.
3° - You have a anti-virus installed in our computer?
you can try to use malwarebytes or Comodo Cleaning Essencials to check your pc against Virus.
4° - Disable all Extensions you have on opera.
5° - Try to Update Opera to latest version..
Before step 6°, i recommend send a image to show whats happening
6° - You can redefine your opera settings
in this page you have 2 options
First option redefine without lose any history or password
second will redefine your browser completely to default
This Will Cause a Loss Of Your Browser Settings, Cookies, Passwords Saved... (it's a fresh start)