`document.scrollingElement.scrollTop` reports the layout viewport offset, so this is going to be affected by recent changes related to viewport sizing. Likely this was regressed by something and then fixed by something.
Bug 1536717 Comment 8 Edit History
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`document.scrollingElement.scrollTop` reports the layout viewport offset, so this is going to be affected by recent changes related to viewport sizing. Likely the behaviour on this page was regressed by something and then fixed by something.