fix(landing): fix sidebar fixed positioning and mobile subnav layout

Only apply fixed positioning to sidebar when expanded (not collapsed).
Fix mobile subnav to top and compensate TOC popover margin.

Author: maxktz

landing/src/styles/globals.css

@@ -373,15 +373,33 @@ html:not([data-anchor-scrolling]) {
   display: none;
 }
 
-/* Sticky positioning is broken inside the fumadocs grid. Use fixed instead. */
+/* Sidebar: fixed positioning so it never scrolls with content, but only when expanded */
 @media (min-width: 768px) {
-  [data-sidebar-placeholder] {
+  #nd-sidebar {
+    will-change: transform;
+  }
+
+  [data-sidebar-placeholder]:not(:has(#nd-sidebar[data-collapsed="true"])) {
     position: fixed !important;
     width: var(--fd-sidebar-width) !important;
     inset-inline-start: max(0px, calc((100vw - var(--fd-layout-width, 97rem)) / 2)) !important;
   }
 }
 
+@media (max-width: 767px) {
+  #nd-subnav {
+    position: fixed !important;
+    top: var(--fd-docs-row-1, 0px) !important;
+    inset-inline-start: 0 !important;
+    inset-inline-end: 0 !important;
+  }
+
+  /* Compensate for fixed #nd-subnav leaving grid flow */
+  [data-toc-popover] {
+    margin-top: 3.5rem;
+  }
+}
+
 [data-toc-popover] > header {
   position: fixed !important;
   top: var(--fd-docs-row-2, 0px) !important;