.with-sidebar {
  display: flex;
  flex-wrap: wrap;
  gap: var(--gutter);
}

.sidebar {
  --sidebar-width: 30rem;
  /* ↓ The width when the sidebar _is_ a sidebar */
  flex-basis: var(--sidebar-width);
  flex-grow: 1;
}

.sidebar.sidebar--s {
  --sidebar-width: 15rem;
}

.not-sidebar {
  /* ↓ Grow from nothing */
  flex-basis: 0;
  flex-grow: 999;
  /* ↓ Wrap when the elements are of equal width */
  min-inline-size: 50%;
}