The collapsed, icon-only form of .sidebar. Add .sidebar--rail to switch the app sidebar to a --sidebar-w (56px) rail. Links drop their labels; icons center and grow to 20px.
Markup: .sidebar.sidebar--rail → .sidebar-header (brand) + .sidebar-group of icon-only .sidebar-links. Active state via .active, same as the expanded sidebar. Icons inherit color through currentColor (--text-tertiary, --text-primary when active).
Shown with the Shift product icons; the brand mark uses --danger (the palette's warm accent).