Q: Problem with external display

Hello there Tobiahao!

Thanks for reaching out to the Apple Support Communities! It sounds like you are having an issue with the external screen resolution. I know how important it is to be sure the external screen shows the Mac computer screen properly. It looks like what you are seeing could be underscan on the external monitor. Check out this article that talks About overscan and underscan on your Mac, Apple TV, or other display.

Adjust the screen size

When viewing content from your Mac or iOS device on your TV or projector, you can adjust the screen size to fit your TV or projector. You can look for these settings on your Mac, your Apple TV, or in the menu of your TV or projector.

Adjust from the menu on your Mac

While mirroring your Mac to a TV or projector, go to System Preferences > Displays. From here, you can use the slider to adjust the overscan or underscan setting. 

If you see an option to change the screen resolution, you can choose the one that matches the dimensions of your TV or projector. If necessary, check your TV or projector's manual to find the display resolution.

Those steps should allow you to adjust the screen to fit properly on the external monitor. Also, be sure you check with the manufacturer for steps to change the monitor itself if you are not able to make the changes with the Mac in System Preferences.

You can also check out the article to Get help with video issues on external displays connected to your Mac. 

Change your display resolution

 

1. From the Apple menu, choose System Preferences.
2. Click the Displays icon.
3. Click Default for display. If you see more than one resolution listed, choose a resolution and refresh rate. If the display turns off when you change resolutions, press the escape key to undo the change. 
   

Cheers!