I have this issue as well. I’m currently alleviating this with a custom CSS:

nav {
  max-height: 50vh;
  overflow: auto;
}

Compatibility-wise, I don’t know the inner workings of Leap Day, howëver. Browser default scrollbars are also ugly and don’t match the theme.

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

The issue isn't stale and hasn't been fixed - ping to keep it alive in the hope that it will be.

The problem appears to be bad nesting in https://github.com/pages-themes/leap-day/blob/master/_layouts/default.html - the "wrapper" div should be closed after nav and section, before footer.