MacOS – How to make windows save their position on screen after re-attaching a display

Chealion answered this on SuperUser.

I won't copy and paste his answer here, as he got the credit not me.

But basically, what you could do is (quoted from Chealion's answer) :

A second method of arranging the Windows so you can see them (if for some reason they're staying off screen) is to change to the application that owns the window in question, hold the Option key down and choose "Arrange in Front" from the Window menu. It will then arrange all the windows of that application in a cascade from the top left of the screen.

I don't know your iMac spec, but look at Apple knowledge base

With Target Display mode, you can use your iMac (27-inch, Mid 2010) or iMac (27-inch, Late 2009) with Mac OS X as an external display. Connect any computer or other device with a Mini DisplayPort to your 27-inch iMac using a Mini DisplayPort to Mini DisplayPort cable or using a converter that converts other electrical, video, and audio protocols from another source device to Mini DisplayPort compliant signals. (Cables and converters available separately.)