Well, that line [goes back](https://searchfox.org/mozilla-central/rev/5102979279f904f2ef068c8f328712ef45c899a0/layout/style/nsStyleStruct.cpp#1725) to bug 78695, but we don't use `mUserInput` to determine stuff when we frame construct, so I think we should just remove it.
Bug 1693061 Comment 5 Edit History
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Well, that line [goes back](https://searchfox.org/mozilla-central/rev/5102979279f904f2ef068c8f328712ef45c899a0/layout/style/nsStyleStruct.cpp#1725) to bug 78695, but we don't use `mUserInput` to determine stuff when we construct frames, so I think we should just remove it.