fix(sw-viewer): prevent sample dropdown layout shift

packages/sw/src/theme/sw-viewer.css

@@ -293,6 +293,9 @@ button.hm-sw-viewer-header__brand { text-align: left; }
 @media (prefers-reduced-motion: reduce) { .hm-sw-viewer-switch, .hm-sw-viewer-switch__thumb { transition: none; } }
 
 /* ---------- Dropdown ---------- */
+/* The menu renders next to its trigger inside the header flex rows: the wrapper must not become a flex item, or the
+   row gains one gap while a menu is open (file name well shrinks, trigger shifts). The fixed list is out of flow. */
+.hm-sw-viewer-menu-root { display: contents; }
 .hm-sw-viewer-menu {
   position: fixed;
   z-index: 60;