I've gone back and forth on this in my head, but settling on it being a shower curtain for several reasons.
1) I think I can see the top of the curtain here:
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2) At one point I wondered if it was sunshine though a window but I'm thinking that line would be straight, and it's curvy.
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3) The "grout lines" don't line up on what I think is a curtain.
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4) If they were grout lines and that's the corner of the shower, I'd think they'd be more angled something like this:
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5) Lastly... one of those "grout lines"/shower curtain folds looks convex to me, not concave:
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All just my amateurish opinion on shower curtains and grout.