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by Joshua on Friday April 13th 2012, 14:05
What I've found to work pretty good is using an emulated desktop and setting the size of the desktop to the size of one monitor, or if you wanted to, both.
If you are using dual monitors I am just taking a shot in the dark here but you probably use NVidia cards don't you? The proprietary drivers lack the support of the XRandR extension when using TwinView. In addition to that Diablo 3 probably does not support your dual screen resolution, ex. 3840x1080 ( two 1920x1080 monitors ), which prevents you from doing fullscreen or fullscreen windowed options.