Regarding the criteria for what constitutes the "current page", I would guess it makes sense to make it whichever page intersects (or is closest to) the top of the viewport.
Bug 1657515 Comment 2 Edit History
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Regarding the criteria for what constitutes the "current page", I would guess it makes sense to make it whichever nsPageSequenceFrame intersects (or is closest to) the top of the viewport.
Regarding the criteria for what constitutes the "current page", I would guess it makes sense to make it whichever [PrintedSheetFrame](https://searchfox.org/mozilla-central/source/layout/generic/PrintedSheetFrame.h) intersects (or is closest to) the top of the viewport.
Regarding the criteria for what constitutes the "current page", I would guess it makes sense to make it whichever [PrintedSheetFrame](https://searchfox.org/mozilla-central/source/layout/generic/PrintedSheetFrame.h) intersects (or is closest to) the top edge of the viewport.