We [use rounded scroll positions in nsHTMLScrollFrame::ScrollByCSSPixelsInternal])https://searchfox.org/mozilla-central/rev/2fa4750e87d49ec7ad19fa1da167a03ef6d5e954/layout/generic/nsGfxScrollFrame.cpp#5009), it is the cause of 1px difference failure in the wpt added in bug 1874551.
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We [use rounded scroll positions in nsHTMLScrollFrame::ScrollByCSSPixelsInternal](https://searchfox.org/mozilla-central/rev/2fa4750e87d49ec7ad19fa1da167a03ef6d5e954/layout/generic/nsGfxScrollFrame.cpp#5009), it is the cause of 1px difference failure in the wpt added in bug 1874551.