Consider the following lines:

(with all defaults this value is 1024px)


Here's mixins.until:

This all means that with all defaults, there is a gap between 1023px and 1024px where neither "desktop nav" nor "mobile nav" have display: block, breaking the navbar (no navigation possible, burger menu button visible but does seemingly nothing because mobile nav is hidden).

To be between these viewport sizes is easily possible:

• 2560x1440px screen resolution
• 125% OS-level scaling
• snap browser window to side (half of screen)
• chromium shows viewport width to be 1023.20px and nav is seemingly broken (hidden)

Suggested solution: Don't use mixins.until i guess..? Or maybe the - 1px can safely be removed and there won't be overlaps between the sets of styles.