@zombilirkisi said in Some videos won't be viewed in Fullscreen mode.:

I even formatted all my disks and re-installed Windows.

Did you start with a fresh profile of Opera after that too or did you restore all your data from a backup or Opera Sync?

I'd download the Opera installer, launch it, click "options", set "install path" to a folder named "Opera Test" on your desktop, set "install for" to "standalone installation", uncheck "import data from default browser" and install. Test in that Opera, but do not enable Opera Sync, do not install any extensions, and do not enable Opera's adblocker. If you still the issue, I'd mess with opera://flags/#use-angle (put it back at default if it doesn't help). If that doesn't help, I'd goto opera://settings/system, disable hardware acceleration and restart that Opera to see if it helps. I'd also try standalone installations of Opera Beta and Opera Developer from https://www.opera.com/download to see if they also have the problem. If not, maybe something was fixed and the fix just hasn't reached stable Opera yet.