Just adding to this, the height of the body element must also be 100%

Hmm, that wasn't the case on my project. html {height: 100%;} did the trick just fine 🤔

Also in page in section The CSS
You have CSS:

  min-height: 100vh;

But in source you use

  height: 100%;

you also have to set the height of the HTML element to 100%

Thanks @kylegordy, I thought I was losing my mind.

@mrFleshka you can read this article for some context on the difference:
https://philipwalton.com/articles/normalizing-cross-browser-flexbox-bugs/

@philipwalton and there is nothing about vh vs %
I understand height/min-height, but i dont understand why you show in code snippet one version and use another (and show how it really work, like example).