Repro rate: ~5/10
Deleting the config folder and re-configuring Opera from scratch does not resolve the issue
Workaround: press CTRL+W to un-freeze Cinnamon (closing the Inspector window that triggered the issue)
Noticing that you are on ubuntu, I have to ask if you have chromium ffmpeg codecs installed?
If yes, opera will play the videos on youtube in h264, which can be hardware accelerated and not stress your cpu that much. If not, opera will play them in webm which can not be hardware accelerated, so your cpu will have to do the playback and heat up.
You can launch Opera with --show-component-extension-options to see some of the built-in extensions. One of those (maybe Google Hangouts) might be the culprit. But, none of them can be disabled, so you're out of luck. If it's not one of those extensions, it's something built in, so you're out of luck there too.
First of all, this is not solution - it's a workaround.
I'm using Opera 56.0.3051.104 on Xubuntu 18.04 but i think you can apply this on any other version of Linux DE.
You have to modify opera's launcher:
Right click on it, Properties, Pencil icon (edit the currently selected item), Command field (third from above) where opera %U is already written. Overwrite with:
opera --window-position=118,64 --window-size=1682,951 %U
That's actual settings for my 24' display and the Opera window is now positioned on the center of the screen in windowed mode. Of course, you can play with numbers (X and Y) until you get the desired position.