firstVisiblePosition 获取到的总是1

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在滚动后再回滚到列表开始位置，然后调用firstVisiblePosition 获取到的总是1而不是0

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SpannedGridLayoutManager layoutManager = new
SpannedGridLayoutManager(SpannedGridLayoutManager.Orientation.HORIZONTAL, 4);
layoutManager.setItemOrderIsStable(true);
layoutManager.setSpanSizeLookup(new SpannedGridLayoutManager.SpanSizeLookup(new
Function1<Integer, SpanSize>(){
@OverRide public SpanSize invoke(Integer position) {
final int span1 = (position == 0 || position == 6 || position == 13 || position == 5 ? 2 : 1);
final int span2 = (position == 0 || position == 6 || position == 13 ? 2 : position == 5 ? 4 : 1);
return new SpanSize(span1, span2);
}
}));
mRecyclerView.setLayoutManager(layoutManager);
mRecyclerView.addItemDecoration(new SpaceItemDecorator(5, 5, 5, 5));

    mAdapter = new SpannedAdapter(getContext(), mRecyclerView);
    mAdapter.setDatas(ItemDatas.getDatas(60));
    mRecyclerView.setAdapter(mAdapter);