diff --git a/src/components/ui/sidebar.tsx b/src/components/ui/sidebar.tsx
index aa83295d..112ee825 100644
--- a/src/components/ui/sidebar.tsx
+++ b/src/components/ui/sidebar.tsx
@@ -355,11 +355,15 @@ function SidebarSeparator({
   className,
   ...props
 }: React.ComponentProps<typeof Separator>) {
+  // `data-horizontal:w-auto` (not a plain `w-auto`) so it dedupes against the base
+  // Separator's `data-horizontal:w-full`; otherwise both survive tailwind-merge and
+  // the line stays full-width, overflowing the `mx-2` inset (most visible in the
+  // collapsed icon rail, where it spilled past the right edge).
   return (
     <Separator
       data-slot="sidebar-separator"
       data-sidebar="separator"
-      className={cn("mx-2 w-auto bg-sidebar-border", className)}
+      className={cn("mx-2 data-horizontal:w-auto bg-sidebar-border", className)}
       {...props}
     />
   )