Video screen goes diagonal when enlarged to full screen

Go step by step and test.

1. Launch Safari holding the Shift key down. 

    Test.

2. Startup in Safe Mode   https://support.apple.com/lv-lv/HT201262

    Login to your account.

    Lauch Safari.

    Test.

    To exit Safe Mode, click the  in the menu bar and then “Restart”.

3 Enable Develop menu.

Use the developer tools in the Develop menu in Safari on Mac ...

Click Develop in the menu bar.

Make sure no item is selected. If you see a tick mark beside any item, click it to remove it.

Move mouse pointer to "Experimental features and go down to the the bottom of the pop-up/dropdown

and click "Restore Defaults."

4 Remove all website data;

    https://support.apple.com/en-gb/guide/safari/sfri11471/13.0/mac/10.15

5 Update Safari in update is available.

https://support.apple.com/en-us/HT204416

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

Try setting up another admin user account in System Preferences/Users & Groups to see if the same problem continues.

Isolating an issue by using another user account https://support.apple.com/en-us/HT204443

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

Try setting up another admin user account in System Preferences/Users & Groups to see if the same problem continues. Please post back on whether or not this worked. Also try the Safe Mode. Please post back on whether or not this worked.

Isolating an issue by using another user account    

Safe Mode - About.       

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

I find this solution unsatisfactory. I do not want to scale down my monitor resolution and have everything a little bit blurry just to make full-screen videos work properly.

This is also not a Safari-exclusive issue. I have the same problem on Google Chrome.

Are there any other solutions? It looks like a bug in Catalina that has not been stamped out, probably because it only affects old machines.

No anti-virus or VPN.

The problem goes away when I use a brand new user account or boot in safe mode.

Here is the EtreCheck report:

I'm not sure my first post worked. Trying again:

I had same problem after upgrading to Catalina. Solution this works 100%

1.) System preferences

2.) Displays

3.) Resolution --- Change to Scaled 1280 by 720

4.) Save and exit

Problem Solved !

SOLUTION for Chrome: go to settings and search for "hardware acceleration" Turn OFF the toggle to use hardware acceleration.