OS X Mirroring and External Displays

I have a not-superfancy 21″ iMac at home. It’s got an old Dell 24″ DVI display attached with a Thunderbolt adapter. I’m generally a big fan of multiple displays on any computer I’m using, and this works out well when I want it, especially if I’m doing something with VMware Fusion that could use a full extra screen. But here’s the thing: In this particular setting, I don’t need or want the external display all the time. It’s connected via VGA to an old Mac mini that we occasionally use, which complicates things a bit more because of input switching. The display is from something like 2007, so it does nothing automatically.

No problem: If I don’t need it, I’ll just turn it off. However, the Thunderbolt adapter means that OS X always sees the display as present, even if it’s powered off. I find this annoying, especially when I launch an application that remembers it has windows on the external display. I have to turn the display on, make sure it’s on the DVI input, blah blah, just to move the window. It’s a small thing, but the grumpy accumulates.

Today, the obvious solution finally occurred to me: Just turn on display mirroring when the external display is unwanted. Command-F1, or Command-fn-F1 if you have media keys disabled.

This gathers everything to the internal iMac display and means OS X (and I) can just ignore the second display. Easy.