Youtube full screen mode only shows 1/4 of the video

No one should do this. I guarantee this is not the solution.

No guarantee anything if you have no idea. You should reconsider your words, as they are the instructions that Apple have given me after hours of phone support.

PPost he chat thread and I'll believe you. No way Apple would tell you to install an adware cleaner.

If I don't believe it, call Apple support and ask about "ADWARE MEDIC".

Greetings!

It would seem I stand corrected. Apologies. I'll wait for a proper fix however. Not sure how this would be adware related.

If you like, you can try without the ADWARE MEDIC step, starting on step nº 5. But I can't guarantee it works.

Hello everyone,

I had the same issue too until I went to the Apple Store wherein they checked out my settings.

I have find a solution to this issue!
If you have an external wireless mouse or USB, this is something that might cause the issue. To fix the issue, follow the following steps.
Optional:
Restart your MacBook.
Steps:
Go to System Preferences ---> General ------> Show Scroll bars -------> Change to "When scrolling."
Go to YouTube on Safari and the problem should be fixed now! 🙂

Kevin Nguyen and Apple Support

AK,

THANK YOU!!!!!! I am not sure what that does (When Scrolling...), but it worked perfectly - thank you very much. Whoooo hoo.

Thank you