Just started seeing this, every post (even posts like this one without a ToC) have the ToC button now, though it opens a large empty window.
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This one is an easy repro 
I can see it too.
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Thanks for reporting it! I’ve added a fix that should solve it, but let us know if you’re still running into this.
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Seems to work, though there’s still weird quirks it seems.
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can you elaborate on that?
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I was referring to ToC’s other current bugs, such as this one:
https://meta.discourse.org/t/changing-topics-shows-a-toc-button-mobile/288308?u=firepup650
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Reverting updates for now