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When my MacBook is connected to my external 4K screen and dock, it sometimes randomly flashes a white overlay for a short amount of time. It's not a hardware issue, as the white overlay has rounded corners, does not extend all the way to the edges of the screen, and is an animated transparent overlay. It looks like an animation that could be used to indicate a screenshot was just made. It takes less than a second.

I've already checked accessibility settings related to flashing when a notification appears, but all of that is disabled (and it happens when no notifications are coming in). It doesn't seem tied to the application I'm using at the moment and it happens seemingly randomly. I have only seen it happen when connected to this particular monitor and dock. Frequency ranges from 2 times in 5 min to less than once an hour, but rarely more than 10 times over an entire evening.

Any attempt at searching for this leads me to posts about broken screens, sudden white flashes (full screen fully white), or flickering, which this is not. The screen and dock work perfectly well besides these occasional overlays. I would highly appreciate it if anyone could tell me what it means, what the feature is called, and/or how to prevent it.

I have a MacBook Pro with M1 Pro (32 GB Memory), running Mac OS Ventura 13.5.1 (22G90).

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  • Quick test. Switch to Finder, press cmd/opt/shift/B [which isn't a valid command by default] - does it beep or flash?
    – Tetsujin
    Commented Oct 17, 2023 at 6:16
  • Do you have Screen Sharing on?
    – benwiggy
    Commented Oct 17, 2023 at 7:13
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    @Tetsujin cmd+opt+shift+B in Finder does not give a beep nor any flash. If I do this in a terminal I get the "pop/beep" but no flash.
    – Taeir
    Commented Oct 17, 2023 at 9:16
  • @benwiggy Screen sharing is disabled in system settings.
    – Taeir
    Commented Oct 17, 2023 at 9:19
  • OK. I just wondered whether the Accessibility 'flash screen when an alert occurs' had been accidentally enabled.
    – Tetsujin
    Commented Oct 17, 2023 at 9:37

3 Answers 3

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This is an annoying bug. What solved it for me:

  1. Go to System Settings > Accessibility > Audio and activate "Flash screen on warning tone"
  2. Go to System Settings > Sound and play the tone once. It should flash.
  3. Restart your machine
  4. Go to System Settings > Accessibility > Audio audio and deactivate "Flash screen on warning tone"
  5. Go to System Settings > Sound and play the tone once. It should only play the tone.
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  • This did it for me. Looks like a this is a new bug present in MacOS 14.5 where notification sound ignores the accessibilty setting and always flashes the screen until next reboot. Small corrections on step 2 and 5, the panel is called "Sound" currently.
    – silverwind
    Commented Jul 7 at 0:54
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    Instead of restarting macOS, you can resolve the problem by merely restarting the coreaudiod daemon by running: sudo killall coreaudiod
    – fjarlq
    Commented Jul 21 at 4:24
  • Thanks. It worked.
    – Tural Asgarov
    Commented Jul 26 at 19:27
  • works in macOS Sequoia
    – alfianrehanusa
    Commented Oct 10 at 2:33
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It seems like the errors are happening specifically in enabling/using the sound output of the dock. My best guess is that because it's the audio output which is erroring/crashing, the system falls back to doing an error flash instead of the usual error beep.

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  • I've had this exact same issue start happening today. I believe it's the same issue as you due to the audio cues not working, but having the white screen flashes instead. A restart of my MacBook Air M3 seems to have solved the problem. Fingers crossed it doesn't happen again...
    – Daniel Mackay
    Commented May 28 at 9:57
  • You can check your console.app for errors near the visual bell.
    – hym3242
    Commented Jun 15 at 9:07
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It may be the windowserver(SkyLight)'s "visualbell". You can trigger it by pressing ctrl+opt+cmd+shift+.(triggers a sysdiagnose) (or call the private symbol "SLDisplayDefaultVisualBell"). Is it the same as yours?

This visualbell is probably used as notification & error indication when no audio is available or is glitching. I have never seen it triggered any time other than when sysdiagnose is triggered by key chord.

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