Consider giving page_sidebar() and page_navbar() main areas min-width

[gadenbuie]

A rule like this is helpful to ensure that opening the sidebar doesn't overly squish the main content

@media (min-width: 576px) {
  .bslib-page-sidebar > .bslib-sidebar-layout.html-fill-item > .main {
    min-width: max(576px, 100%);
  }
}

This rule isn't quite perfect because the scrolling happens across the entire content area and not the main sidebar area, but it's a reasonable starting place.

Collapsed

Expanded, with rule

Expanded, without rule