kylinScrollView.getScrollView().setOnScrollChangeListener(new NestedScrollView.OnScrollChangeListener() {
            @Override
            public void onScrollChange(NestedScrollView v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
                if (scrollY >= cTab.getTop()+contentView.getTop()){
                    tTab.setVisibility(View.VISIBLE);
                }else {
                    tTab.setVisibility(View.GONE);
                }
            }
        });

android:fillViewport="true"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"