Bug in nav-collapse margin-inline sizing

[janbrasna]

Steps to reproduce the issue

1. Open https://demo.fractal.build demo site on desktop and start making the window smaller
2. Once it hits breakpoint @include mq($until: nav-collapse) the menu disappears
3. However the main content is covered or cut off

Screen.Recording.2023-12-26.at.0.03.28.mov

Reproduces how often:

100%

Reduced test case

can be reproduced live, in official demo, and many showcase implementations (ghent, mozilla etc.)

Context

• [docs] Resize issues when navigation is open mozilla/protocol#902

[janbrasna]

I believe there's some flawed logic in

fractal/packages/mandelbrot/assets/scss/core/_mixins.scss

Line 136 in dd6cd93

@mixin margin-inline($position, $amount) {

where some of the amounts should be negative, or maybe the whole argument here:

fractal/packages/mandelbrot/assets/scss/components/_frame.scss

Line 29 in dd6cd93

@include margin-inline(end, $width-sidebar);

might be * -1 negative to fix this.