Weird Glitch When Double Clicking Videos on Youtube to Go into Fullscreen on Safari

Hello Tuna_28x, 

Thanks for reaching out on the Apple Support. From your message it appears that you're unable to have a YouTube video enter full-screen mode when you double click on it. 

I tested this on my iMac using an Apple Support video on YouTube, and was able to enter and exit full-screen mode in Safari. When you click on the full screen icon in the bottom right corner of the YouTube video window, does it enter and then exit full-screen mode? This will help isolate the issue. If that works as expected, work through these troubleshooting steps:

1 - Quit all apps. If an app will not quit, follow these steps to force it to quit: How to force an app to quit on your Mac. 

2 - Restart your Mac: Go to the Apple menu in the top left corner of your screen > Restart. 

3 - Hold down the shift key on the left side of the keyboard as you click on the Safari app to launch it. This allows the app to bypass the last saved state for the app. When the app begins bouncing in the dock, you can release the shift key. Once Safari is up and running, see if there is any change when you try to view this Apple Support YouTube video: 10 helpful Apple Watch tips you should know - Apple Support - YouTube.  

4 - If you're seeing the same results, restart in safe mode and test again: How to use safe mode on your Mac. 

5 - If still occurring in safe mode you’ll need to create a new user, sign into that new user with your information, and test it out again. This article will show you how: How to test an issue in another user account on your Mac.

All the best. 

Are you running any anti - virus programs? Using a VPN?

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply. After pasting the report in a PasteBin page, go to the top of the page, and copy the address in the URL bar. Paste that in a new reply.        Pastebin.    

Etrecheck – System Information.      

Web site version if required.  https://etrecheck.com