Shortly after logging in there several frames without a tli, and so no scene is
rendered on them. Since we glClear the entire buffer each frame and re-render
the most recent scene draw to emulate the vanilla effect of not painting over
the viewport, the viewport offset x/y values at the time we draw the frame may
be -1 due to that frame not having the scene rendered. Store the viewport
offsets at the time of the most recent scene draw and use them for each
subsequent frame.
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