Prevent flashing during navigation

assets/style.css

@@ -48,6 +48,28 @@
     --reading-width: 60rem;
 }
 
+/* Cross-document view transitions: page navigations stay full reloads, but the
+   browser holds the old frame until the new page is ready and swaps them
+   without a blank flash. The header and tree rail are named so they swap in
+   place (no crossfade) — only the rest of the page fades, and only briefly. */
+@view-transition { navigation: auto; }
+body > header { view-transition-name: app-header; }
+.tree-sidebar { view-transition-name: tree-sidebar; }
+::view-transition-group(*),
+::view-transition-old(*),
+::view-transition-new(*) { animation-duration: 0.12s; }
+::view-transition-group(app-header),
+::view-transition-group(tree-sidebar),
+::view-transition-old(app-header),
+::view-transition-new(app-header),
+::view-transition-old(tree-sidebar),
+::view-transition-new(tree-sidebar) { animation: none; }
+@media (prefers-reduced-motion: reduce) {
+    ::view-transition-group(*),
+    ::view-transition-old(*),
+    ::view-transition-new(*) { animation: none; }
+}
+
 /* Anchor / TOC jumps are handled by the .center scroll container's own
    scroll-padding-top (the header no longer overlaps content in the app-shell),
    so no html-level scroll padding is needed. */