I'm on Kubuntu 18.04 (KDE Plasma Version 5.12.6) with the "Breeze" theme and "Breezemite" window decoration, this is the "Noto Sans" font. With the Latte Dock on, looks a lot like a mac.
Try removing chromium ffmpeg codecs and then check how good or bad vivaldi can perform on h264 content. I am 110% sure, like opera, it won't play a thing. And no idea why it works with newer versions of it
I would do the above for you, but I do not use ubuntu, thus no chromium ffmpeg codecs at all, and I do not use electron based crap like vivaldi (or skypeforlinux, vs code, atom etc).
On the other hand, how hard can it be for someone to make a small script that (in this specific order)
checks the installed version of opera
"calculates" the proper chromium version it is based on
downloads the correct chromium ffmpeg codecs package from that ubuntu ppa
extracts the libffmpeg from it
places it in the directory opera's libffmpeg is, replacing the original one
I do not do scripting, I am bad at it, but I am sure someone can do it. And the same thing for widevine, except it would download google chrome...
It would be a bit more helpful if you posted what opera outputs when it is launched from the terminal.
Btw, ubuntu 18.04 has kernel 4.15 by default and it will probably stay with that one until its eol in 2023, so you'd better not install kernels from third party repos if you prefer stability over features.
From the way I understand it, the libffmpeg.so file that Opera uses doesn't contain the proper codecs for certain videos. This is how I fixed it for KDE Neon (Ubuntu variant). Make sure you have the Chromium browser because this will break Opera in the process and you will need the Chromium libffmpeg.so file. Use this at your own risk.
sudo updatebd (update locate database if you never used it before)
sudo locate libffmpeg.so (shows location of all libffmpeg.so files)
sudo rm /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so (removes the libffmpeg.so that Opera is using, if your results are different from the locate command then use that path)
sudo cp /usr/lib/chromium-browser/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/ (copy the Chromium libffmpeg.so into the Opera directory)
I have Opera 54.0.2952.64 on Manjaro XFCE
I get similar issue with window buttons not shown when maximised
There is a close button to right of tab menu, but nothing is visible except a hover tootltip "Close"
Double clicking in blank space to left of tab menu will restore window size