This is how it looks here:

And on developer 46:

What's your OS? I'm on Windows 10.

I'm also on win 10.

I cannot replicate, but why would enabling Cinema mode not achieve what you want? The value appears to be saved for the current session only, but entering the below from console will make it permanent, although it may not be what you are looking for if you do not wish to use Cinema mode for some reason.
document.cookie="wide=1; expires=Tue, 19 Jan 2038 04:14:07 UTC; domain=.youtube.com";

Yeah that seems like a temporary fix, but when if you're entering private window it will revert back to the small screen.
I know it may sound trivial but it is annoying having to press the cinema mode every time.

  Are both in theater mode? 
    

Neither of them are in theater mode.

The one on the left seems to be, that's why I asked.

No, I'm 100% sure neither are in theater mode...

Are both in theater mode?

Neither of them are in theater mode.

It seems OK here. Can you share a screen-shot?

I managed to revert to Opera 44 to show differences in youtube video player:

http://imgur.com/a/mlJyB

After the Opera 45 update the video playback on youtube seems to be scaled wrongly, before update it was fine, now the youtube player screen is much smaller than it used to be (No sidebars enabled and zoom is 100%).

Please fix this bug.

p.s. apart from the above It was a very good update.

Hello, this is a message to the developers, could they please fix the bug that stops the icons of the extensions not showing up in the private window even though I enabled the extension in private. Thanks.

I'm sad to say I'm getting increasingly frustrated with Opera to the extent I'm about to give up.
I like the VPN but it's annoying that when I switch it on and off, it resets the Plugin setting to "Click to Play". I now can't play videos on Twitter at all, it simply gives the message "The media could not be played".
I'm also getting regular page crash issues - disabling all extensions seems to make no difference.
I'm using version 0.0.2308.81 on Windows 10, though if these issues can't be resolved I'm switching to Chrome

I had this problem but I recently updated to 40.0.2308.90 and it seems to fix it. Can you check again?