fix: mobile hub AI keyboard layout and instance top nav scroll

Sync hub shell to visualViewport when the keyboard opens to prevent white screen above chat input. Make instance tabs horizontally scrollable with active tab centered on phone.

Co-authored-by: Cursor <cursoragent@cursor.com>

static/instance_theme.js

@@ -251,6 +251,32 @@
     });
   }
 
+  function initMobileTopNav() {
+    const mq = window.matchMedia("(max-width: 720px)");
+
+    function scrollActiveTab(nav) {
+      const active = nav.querySelector("a.active");
+      if (!active) return;
+      requestAnimationFrame(() => {
+        try {
+          active.scrollIntoView({ inline: "center", block: "nearest", behavior: "instant" });
+        } catch (_) {
+          active.scrollIntoView(false);
+        }
+      });
+    }
+
+    function apply() {
+      if (!mq.matches) return;
+      document.querySelectorAll(".top-nav").forEach(scrollActiveTab);
+    }
+
+    apply();
+    mq.addEventListener("change", apply);
+    window.addEventListener("resize", apply);
+    window.addEventListener("orientationchange", apply);
+  }
+
   function initFromHubMessage(data) {
     if (!data || data.type !== "hub-theme-sync") return;
     if (!isHubLinked()) return;
@@ -285,6 +311,7 @@
 
     const onReady = () => {
       initToggleUI();
+      initMobileTopNav();
       syncInlineStyles(get());
       patchHubNavLinks(get());
       observeDynamicLists();