Tooltips are given to us in Gecko coordinates, and we aren't account for scaling correctly, so they show X times too far down and to the right (where X is the scale factor). I've found that tooltips and hover effects are unreliable in general under scaling (which would be another, probably core, bug, that I haven't pinned down and filed yet); for testing this issue, I've found that starting a drag of the image works well to force the tooltip to display.
Actually, we aren't doing anything wrong; Gecko is giving us the wrong values. The expectation is that nsITooltipListener::OnShowTooltip would give us Gecko coordinates (meaning scaled), but instead we get a point that is device pixels from the upper left corner of the ChildView. I would guess that it's nsChildView::WidgetToScreen, since that definitely makes bad assumptions about window/screen vs view coordinates, but since none of Cocoa Widgets has been audited for this kind of problem it's hard to be sure.
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