Hi @istservice, thanks for the good question. When using 100vh and fixed position on the navbar, I would suggest to apply the fixed position not to the navbar itself, but to a parent div first, then drop the navbar in that parent div block.
Here is a video:
https://cl.ly/3I3p121a1M2v/Screen%20Recording%202018-03-20%20at%2009.35%20AM.mov
I hope this helps.