Restore hub iframe soft nav to cut blank tab switch gap.

Use fetch in-frame navigation with overlay and hover prefetch; show delayed hub loading spinner instead of hiding the iframe.

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

manual_trading_hub/static/app.js

@@ -429,9 +429,24 @@
     return j.url;
   }
 
+  /** @type {number | null} */
+  let instanceFrameNavLoadingTimer = null;
+
   function setInstanceFrameNavLoading(loading) {
     const shell = document.getElementById("instance-frame-shell");
-    if (shell) shell.classList.toggle("is-instance-nav-loading", !!loading);
+    if (!shell) return;
+    if (instanceFrameNavLoadingTimer != null) {
+      clearTimeout(instanceFrameNavLoadingTimer);
+      instanceFrameNavLoadingTimer = null;
+    }
+    if (loading) {
+      instanceFrameNavLoadingTimer = window.setTimeout(() => {
+        shell.classList.add("is-instance-nav-loading");
+        instanceFrameNavLoadingTimer = null;
+      }, 140);
+      return;
+    }
+    shell.classList.remove("is-instance-nav-loading");
   }
 
   async function openInstance(exchangeId, nextPath, opts) {
@@ -3354,7 +3369,7 @@
         if (!d || typeof d !== "object") return;
         if (d.type === "instance-frame-navigating") {
           setInstanceFrameNavLoading(true);
-        } else if (d.type === "instance-frame-ready" || d.type === "instance-theme-ready") {
+        } else if (d.type === "instance-frame-ready") {
           setInstanceFrameNavLoading(false);
         }
       });