[Repo Assist] Experiment: View Transition API for smooth cross-document page transitions (issue #1077) (#1080)

* Experiment: add View Transition API CSS for cross-document page transitions (issue #1077)

Enable smooth page transitions using the View Transition API:
- @view-transition { navigation: auto; } opts all pages in
- Named #content with slide-in/slide-out animation on navigation
- header, #fsdocs-main-menu, #fsdocs-page-menu update instantly (no fade)
- prefers-reduced-motion: all transitions disabled

Browser support: Chrome 126+, Safari 18.2+. Gracefully ignored elsewhere.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

docs/content/fsdocs-default.css

@@ -1294,3 +1294,72 @@ dialog {
         opacity: 0.75;
     }
 }
+
+/* =========================================================================
+   View Transition API — cross-document page transitions (experimental)
+   Supported in Chrome 126+, Safari 18.2+. Gracefully ignored elsewhere.
+   ========================================================================= */
+
+@view-transition {
+    navigation: auto;
+}
+
+/* Isolate the main content area so it can slide independently */
+#content {
+    view-transition-name: fsdocs-content;
+}
+
+/* Keep persistent navigation panels static (no animation) */
+header {
+    view-transition-name: fsdocs-header;
+}
+
+#fsdocs-main-menu {
+    view-transition-name: fsdocs-main-menu;
+}
+
+#fsdocs-page-menu {
+    view-transition-name: fsdocs-page-menu;
+}
+
+@keyframes fsdocs-slide-in {
+    from {
+        opacity: 0;
+        translate: 0 12px;
+    }
+}
+
+@keyframes fsdocs-slide-out {
+    to {
+        opacity: 0;
+        translate: 0 -8px;
+    }
+}
+
+::view-transition-new(fsdocs-content) {
+    animation: fsdocs-slide-in 200ms ease both;
+}
+
+::view-transition-old(fsdocs-content) {
+    animation: fsdocs-slide-out 150ms ease both;
+}
+
+/* Nav panels: instant update (old fades out instantly, new fades in instantly) */
+::view-transition-old(fsdocs-header),
+::view-transition-new(fsdocs-header),
+::view-transition-old(fsdocs-main-menu),
+::view-transition-new(fsdocs-main-menu),
+::view-transition-old(fsdocs-page-menu),
+::view-transition-new(fsdocs-page-menu) {
+    animation-duration: 0s;
+}
+
+/* Respect user's reduced-motion preference */
+@media (prefers-reduced-motion: reduce) {
+    ::view-transition-group(*),
+    ::view-transition-image-pair(*),
+    ::view-transition-old(*),
+    ::view-transition-new(*) {
+        animation-duration: 0s !important;
+    }
+}