Thin white lines flashing across the monitor.

What size, make and model is the Display...?

Test a different HDMI cable. Preferably a short 3.3' high speed certified 4K or better HDMI 2.1 cable.

for example: > https://www.amazon.com/dp/B0CQP27WZ2/ref=sspa_dk_detail_1?

You're welcome.

That HDMI cable is overkill for the other devices (switch, xbox, old Mac) that you mentioned, but necessary for a picky newer Mac and an up-scale 4K or better displays.

Speaking of displays... You never answered the make and model question about your display. The reason I asked that question first, is because you might have better luck using a USB-C to HDMI cable or a USB-C to DisplayPort cable if that display supports DisplayPort input.

"Thin white lines flashing across the monitor.: I have a M1 Mac mini 16gb ram and I use a Nvidia monitor. recently I have been getting very thin white lines flashing across the screen. if resolution matters for this I have it in 1440p. I use a good HDMI that I have used with other devices (switch, xbox, my old Mac.) so could this be a Mac problem that means I need to get it to the Apple Store asap? or could I maybe fix it at home? I am also running macOS sequoia as if I upgrade to Tahoe the thing will become a paperweight XD"

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White Lines on a Screen:

What's the year of your Mac? Go to: Apple menu > About this Mac. This may all come down to using a 4K cable. So is it 4K? As an example, here it is for a 2024 Display support: Mac mini (2024) - Tech Specs - Apple Support

Reply back, and let up know if this does the fix. If so, informing others may help many out these with this issue,