🐛 Let the fullscreen mobile menu escape the floating header

The floating header's backdrop blur makes the pill a containing block
for fixed-position descendants, so the fullscreen mobile menu rendered
at the pill's height instead of filling the viewport. Lift the filter
while the menu is open. Fixes #3057.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/css/compiled/main.css

@@ -4900,6 +4900,10 @@ body:has(#mobile-menu-toggle:checked) {
   visibility: hidden;
   transition: visibility 0.3s, opacity 0.3s ease-in-out;
 }
+header > div:has(.mobile-menu-fullscreen #mobile-menu-toggle:checked) {
+  -webkit-backdrop-filter: none;
+  backdrop-filter: none;
+}
 .nested-menu:nth-last-child(-n + 3) .menuhide, .mobile-header-row .menuhide {
   inset-inline-end: 0;
 }

assets/css/main.css

@@ -485,6 +485,15 @@ body:has(#mobile-menu-toggle:checked) {
     opacity 0.3s ease-in-out;
 }
 
+/* A backdrop-filter turns the header pill into a containing block for
+   position:fixed descendants, which would trap the fullscreen mobile menu
+   inside it. Lift the filter while that menu is open — the fullscreen
+   overlay covers the header anyway, so the change is invisible. */
+header > div:has(.mobile-menu-fullscreen #mobile-menu-toggle:checked) {
+  -webkit-backdrop-filter: none;
+  backdrop-filter: none;
+}
+
 /* Dropdowns near the end of the bar open leftward so they stay on screen */
 .nested-menu:nth-last-child(-n + 3) .menuhide,
 .mobile-header-row .menuhide {

layouts/partials/header/components/mobile-menu.html

@@ -1,5 +1,5 @@
 {{ $mobileMenuStyle := .Site.Params.header.mobileMenuStyle | default "fullscreen" }}
-<div class="mobile-header-row relative flex items-center h-14 gap-4">
+<div class="mobile-header-row{{ if eq $mobileMenuStyle "fullscreen" }} mobile-menu-fullscreen{{ end }} relative flex items-center h-14 gap-4">
   {{ if hugo.IsMultilingual }}
     {{ partial "header/components/translations.html" . }}
   {{ end }}