In mirror mode, both displays must have the same resolution. If both displays have the same mode, then they will both be able to display full screen and the ASUS monitor will upscale the image.
If the mode is only available in the primary monitor and not the ASUS, it will still appear in the mode list, but we will actually choose the next highest resolution (as we don't support the resolution requested) and "letterbox" the image. This is what is happening at 1366x768.
Wim
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