scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp: moves the delegate
from the old scrollerImp to the new one, and also happens to call shouldUseLayerPerPartForScrollerImp.
Since scrollerImpWithStyle: has not returned yet, the scrollbarPainterForScrollbar still returns the old scrollerImp,
so this assertion fires. It's safe to remove this because supportsUpdateOnSecondaryThread doesn't make use of the imp.
Created attachment 229937 [details]