Why am I seeing subtle horizontal lines? (Intel HD Graphics 4000) I can't take screenshot of it and a picture taken with an iPhone doesn't show it, but here is an exaggeration of what it looks like (visible all over the screen):



It appeared very recently; probably linked to upgrading to Mojave a few days ago.



Any solution that doesn't require a visit at the Apple Store / a repair shop would be greatly appreciated. Thanks.
As a general guide, if it doesn't show up in a screen-shot, it's a hardware fault.

Potential fail-points are the monitor's video cable, the screen, or the graphics chip itself.
Unless you fancy stripping it apart yourself to try to re-seat or replace the video cable, I think a trip to the repair shop is on the cards.

iFixit do some good teardown &amp; repair guides, if you want to tackle something like this yourself - MacBook Pro Repair - though you'd need to see if they have one for your specific model.