Did you find anything @Brian G? The same window has popped up 3 times today.
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What I have noticed is that it only seems to happen on posts that don't already have a like.
No clue as of now.
Over the past few days while on Chrome on Windows occasionally the message/dialogue/reply/whatever-you-want-to-call-it box is just totally missing:
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As opposed to the same view in Internet Explorer:
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I believe that may have something to do with similar posts addon. If you refresh, should pop back up.