Steps to  fix your Mac's glitchy screen:

• Restart your computer.-Restart by going to the Apple Menu > Restart. Alternatively, turn it off and wait a few minutes before turning it back on.
• Update your Mac's operating system.-Go to the Mac App Store or select System Preferences from the Apple Menu. If a macOS update is available, install it. Back up your data in advance to an external hard drive, a third-party cloud storage service, or iCloud. Now, follow the instructions to upgrade macOS, and once your Mac has restarted, check to see if the screen flickering issue is still present under the same circumstances as before.
• Clear your caches and garbage files on your computer.-To address this issue, you can download an app that cleans up a lot of system trash that slows things down. CleanMyMac X is available for free download.
• PRAM should be reset.
• Change the Energy Saver settings as needed.-If you have a MacBook Pro, the screen blinking can be caused by the Energy Saver not operating properly. Your Mac will switch between two distinct graphics chips if you enable 'Automatic graphics switching' in Energy Saver. However, things do break. Go to System Preferences > Battery and select it .'Automatic graphic switching' should be unchecked.

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