Some time ago, the automatic detection of external displays stopped working when plugging in a display via Mini DisplayPort to my 13", late 2011 Mac Book Pro with Lion installed.
This is regardless of whether the external display is connected via DVI or VGA, and it doesn’t work either when I plug a display in, nor when I remove it later.
I can still go to the Displays prefpane and manually “Detect Displays”. But I can only do that if I’m actually seeing something on my display, which isn’t the case when I’ve just unplugged the MBP from an external display that I used as a primary screen: the MBP continues showing only the secondary screen. I have to re-plug it or restart the laptop to get full vision back.
What’s more, I also have an external display connected via USB/DisplayLink and here, automatic display detection works. Only the Mini DisplayPort acts up. (But I’m using several different monitors which all exhibit this problem so it’s probably not the fault of the monitors.)
Is this a known problem? Could this be a mechanical defect? (But then, why does it find a connection at all?)