Thanks everyone for helping me.
For now - it is working just fine. I don't know if its an effect of some kind of update or something else.
/Thank You again!

@leocg yes, ive tried it. What is more interesting On Fox Browser ... same issue. Only EDGE is ok. So it is obvious that problem is with browser, not in system or graphic card

@silentslayer So ... as for now there is no solution for this

Hello,
Today, when i opened Youtube on Opera GX - its size doesnt match my resolution ( =yesterday everything was fine ).
=I cleard cookies/ cache/ all history - doesnt help
=I restarted system/ updated browser - doesnt help

What else can i do .. And why i have such weird "thing" ?
/Thanks in advance for helping me

Hello there!
Today when i wanted to watch YT video i had ( and still have ) weird problem.
To be more specific - as U can see in image - video size doesnt match browser size. I do not know why this happened, but it is pretty werid and annoying.
=Did U have such problem? Any solutions? ( I've updated broswe/ i've cleard cookies/ all history - nothing helped )
/Thanks for any help