flex-item does not have "min-width:0" In this case, the width of content in the inner box will be the minimum width of flex-item. So flex-item will not try to shrink the inner box.
flex-item has "min-width:0" In this case, flex-item can shrink up to 0px. flex-item will squeeze the inner box. The inner box will show a scroll bar because it has "overflow:auto"