Hi @nvmjustagirl

i played around with some settings.
What is did:

install opera 64bit. (it's not that easy to find on Opera site) set opera to use Nvidia card

This already decreased the problem.

Then, I disabled my (only) plugin:
'Ublock Origin' >> instantly the flickering and content mixing between Opera windows is gone, NICE!
Next, I tried disabling the build-in spam blocker to check if these two were conflicting with each other. but that was not the case.

Will do some testing later, as my current (second) monitor setup will not exhibit this odd behavior.