MacBook Air M4: White lines on top border appear & disappear

"MacBook Air M4: White lines on top border appear & disappear: small white lines on top border which goes away and come back when clicking on menu model - MacBook Air M4 OS - 26.2 (25C56) And this issue not happening when use full screen apps, for eg playing video in vlc in full screen. I have made a video describing the issue."

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Thanks for the video.

Lines Reappearing When Overing Over the Dock:

A. Force Kill the Dock:

Using Terminal, see what happens when you Force Kill the Dock. See if it comes back, when you restart. If so, it's a bug, and you need to report it.

1. Go to: Applications
2. Select: Utilities
3. Open: Terminal
4. Use this Command:

killall Dock

B. Create New User:

For temporary use, create a new Administrator. See what happens when you log in to it. Do you still get this issue? If not, then is it a misconfiguration in the current user. If so, then you'd likely have to resinstall the macOS.

C. Reinstall the macOS:

Go Here: How to Reinstall macOS from macOS Recovery - Apple Support. Important: Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall. Once reinstalled, try this once more

D. A Bug in the Update:

It might be a bug in the update. Apple might come out with a fix, but they'll only know of this if it is reported.

E. Report this to Apple:

Do your Part: Report this to Apple. Apple may not get back to you directly, but the more feedback Apple receives on this, the more they will know what is going on, and what needs to be fixed in upcoming updates. These are just user-to-user forums. Apple will you nothing of your findings, if you don't provide them to them.

1. Go Here: Feedback - macOS- Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Select: "User Interface" for the "Feedback Area"
4. Comment: when this all started, and how you have troubleshot it thus far. Include the URL to this thread.
5. Proceed from there as necessary

"I tried the `killall Dock` command. It just flickers the screen quickly and for a moment white lines goes away and then come back. please see the video I captured."

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Thank you for the video.

Lines Reappearing When Overing Over the Dock:

A. Screen Record:

Record your screen, and see if it then show, when you play it with the Photos app. If seen, then it is a software issue. If not seen, then it is a hardware issue, and you should ge this serviced.

B. Change the Background to Solid White:

Change the background to solid white. See if this occurs. In contrast, that would indicate an issue with the GUI. A reinstall should fix that. See where things go from there.

Reinstalling the macOS:

Go Here: How to Reinstall macOS from macOS Recovery - Apple Support. Important: Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall. Once reinstalled, try this once more

"I have too many things over there to ditch that and switch to new user. Any way we can reset the user without loosing too much of customization?"

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Excellent job troubleshooting this.

If it were me, I would procede with the reinstall. It will take some time; just be certain yo back up first, so that nothing is lost.

Reinstalling the macOS:

Go Here: How to Reinstall macOS from macOS Recovery - Apple Support. Important: Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall. Once reinstalled, try this once more

I think it's just the new Liquid Glass appearance of Tahoe 26. If you prefer you can change from 'tinted' to 'clear' or vice versa under settings > appearance > liquid glass.