Picture below is showing my problem:
So my question is, how I could implement Bootstrap 4 navbar which is actually working and using flexbox? I guess easiest fix is to stop Bootstrap adding that extra style to element. It's not needed since if that element is not visible, then it has "display: none" anyway. And if it's visible, height shouldn't be certainly zero.
This problem was fixed in Bootstrap 4 Alpha 5 release. So updating Bootstrap solved the problem.