高度自适应时，css如何写可以使滚动条显示

html

    

        xxxxxxxxxx.....
    

css

.parent{
    display: flex;
    flex-direction: colum;
    max-height:90%;
 }

.child{
    flex: 1;
    height: 0;
    display: flex;
    flex-direction: column;
    overflow: auto;
 }