I've been getting something similar on one of my toons. I only get it on my druid, in cat form and it's just to the right of my energy bar. Mine appears about half the width of yours though, and I do believe is there in windowed or full screen. As soon as I change out of cat form it goes away. IIRC I didn't have it on the early alphas and it just appeared within the last few releases.
--W
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