Currently for G-Sync to engage when an OpenGL application is open, it must be able to flip, which requires that its window be unoccluded, unredirected, and most importantly covering the entire desktop. In situations were your X Screen spans multiple monitors G-Sync will stop functioning as a Fullscreen application no longer has coverage of the entire X display. Disabling the second (or all other) monitor(s) will return functionality.
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