Commenting script?

I'm hoping someone can help me locate an AO3 script or add-on I heard about a while ago. It provides a hovering text box where you can draft a comment as you read. I can't remember the name of it. Does anyone know where I can find this? I checked the AO3 FAQ, but it's not among the third-party tools listed there, and my searches of DW and Tumblr haven't turned up anything. Any ideas?

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