When you run multiple displays of different resolutions on Mac, Mac appears to "pretend" that all of the monitors actually have the same vertical resolution by creating "virtual" display realestate to fill in the gaps between each of the smaller displays and the largest display. For example, I have a macbook pro and a 27" Cinema display.
I run my Cinema display at its native resolution, 2560x1440, and the my macbook at the far-right, "more space" setting (it's smaller than 1440, but it doesn't tell me exactly what it is). Because of how I have my monitor and laptop situated on my desk, I use the "arrangement" tab in system preferences to align the bottom edge of the displays. When I do this, there is a section above the macbook screen that is shown as blank or empty in the arrangement window.
HOWEVER, I discovered that if I rearranged things such that the top edges of my displays were aligned, my missing application window was quietly waiting for me at the top-left of the laptop monitor space, thinking it was on-screen and visible the whole time. From there, I discovered that by moving my laptop screen back down to align with the external monitor at the bottom, the system was kind enough to force the now hidden window to the top left of my Cinema display.
also see the very popular queestion What Window Management Options exist for OS X?
– cwd – 2014-09-08T23:00:53.193You can check out http://serverfault.com/questions/7237/retrieving-windows-stuck-on-external-monitor-os-x on Server Fault as well.
– Chealion – 2009-07-15T15:27:45.403