Once you see glitchy behavior, your first course of action should be to examine your phone for external damage. It could be that a cracked or chipped screen is causing your phone to act up. Take off screen protectors or stickers and make sure the display is clean, as debris on it may register as touches and cause problems. If none of these factors seem to be causing the issue, then it might be possible that the flickering display is not related to an obvious instance of surface damage.
In some cases, a flickering or flashing screen can be caused by issues with the hardware inside of the phone, in which case you’ll likely need to take the handset to a professional to have it serviced. If you’re not afraid of opening up the device (and doing so won’t void any warranties you may have), then consider removing the back of the phone and carefully checking the display flex cable to see whether it is loose or damaged. Android forums are full of threads from users who narrowed their display issues down to a damaged flex cable or loose connector.
This should be a last resort, however, as it’s likely the issue is due to a software bug, malfunctioning app, or your phone’s settings. Your first troubleshooting attempt should involve rebooting your phone. Turning a phone off and then back on is a common way to fix many problems, including annoying crashing issues, and it’s the fastest troubleshooting method. On most Android devices, simply hold the power button for several seconds to power it off, and then long press it again to turn the phone back on.
Source: https://www.slashgear.com/962015/how-to-fix-a-flickering-or-flashing-android-phone-screen/