A mysterious blank patch on my screen

I moved house recently, and while setting my Mac up I hadn’t put the Thunderbolt cable for the second screen in securely. It slipped out, and when put back in there was a large black window across most of the two screens. If I have open applications in front of the black area, they’re fine unless I click on the black window. Then they disappear behind it. I’ve tried Force Quit, but only the other open apps appear in the list.

A partial workaround is to shut down with ‘Reopen windows on restart’ unchecked. However, the next time I shut down with ‘Reopen windows on restart’ checked, it reappears. Any help? Steve Mapstone Well… this is a weird one. Black rectangles on the screen can sometimes mean faulty RAM, and during a house move it’s common for RAM modules to come loose. But if the RAM were loose or faulty then you’d have a permanently dead area on the screen, not one that you can slide applications over.

Just because you don’t see an application in the Force Quit list that appears when you hit ç+å+oe doesn’t mean you don’t have something running. Try opening Activity Monitor and use the X icon in the top-left to close each process that shows your login name in the ‘User’ column. If the black rectangle disappears at any point then you have a buggy app that’s displaying a blank window somehow.

I think it’s more likely that the saved window state has got corrupted. This will have been completely unrelated to your loose cable – that was a coincidence. Deleting ~/Library/Preferences/com.apple. loginwindow.plist and restarting ought to be enough to cure this. If for some reason it isn’t, then you may have to escalate to a full reinstall. Back up your data and documents, and do a clean reinstall from the Recovery partition. Don’t restore your entire disk directly from Time Machine, because you risk restoring whichever corrupted preference file is responsible. Restore apps one at a time or download them afresh, to be on the safe side.