Setup a CSS variable for the header height by adding [--header-height:--spacing(14)] to the body or page container and use h-(--header-height) to set the height of your header. This variable is then used to offset the sidebar to account for the sticky header height.
Use the inset variant on hlm-sidebar and the hlmSidebarInset directive on your main element to create an inset sidebar layout. Both elements must be siblings for the styles to apply correctly.