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Question: High Sierra: Second Monitor Trouble

When I upgraded to High Sierra, my second monitor began to misbehave. It had been fine for years under all the old OS versions. (It's VGA connected via my iMac Thunderbolt port w adaptor.) Sometimes it works fine, but other times it briefly shows the icons on my extended desktop and they then blink off and the screen dims to gray; I can still see the menu bar, but I can't move a window to that screen, nor can I draw a rectangle by dragging and holding on the screen. My calendar program opens on that screen and that works fine, but the desktop behind it is effectively dead. I can make this problem go away by briefly mirroring displays. It sometimes re-occurs when I wake the system from sleep.


Any ideas?

iMac, macOS High Sierra (10.13.3)

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Mar 3, 2018 10:13 AM in response to tomde In response to tomde

Hello tomde,

Thank you for using Apple Support Communities!

I understand from your post that after updating to macOS High Sierra that your external monitor started having issues. The following article has steps which may help resolve this type of issue:

Get help with video issues on external displays connected to your Mac - Apple Support

Best Regards.

Mar 3, 2018 10:13 AM

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Mar 3, 2018 4:25 PM in response to becky_s22 In response to becky_s22

Thanks, Becky. I had done all of those things. Apropos: virtually nothing in the linked article pertains to a setup which had worked correctly under Sierra, and then ceased to work under High Sierra. If you look at the long list of unsolved High Sierra + dual monitor problems on this forum (and elsewhere) you are led to believe that there is simply a bug in the current release. I think that's what causes my problem. I'm lucky that I can kludge to make it work by briefly mirroring monitors.

Mar 3, 2018 4:25 PM

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Question: High Sierra: Second Monitor Trouble