fix(hub): eliminate iframe flash when switching instance nav tabs

Use soft in-frame navigation and loading overlay in hub instance shell; pass embed=1 for iframe SSO opens.

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

static/instance_theme.js

@@ -283,10 +283,92 @@
     apply(data.theme, { skipStore: true });
   }
 
+  function notifyParentFrameNavStart() {
+    if (!isHubLinked()) return;
+    try {
+      window.parent.postMessage({ type: "instance-frame-navigating", theme: get() }, "*");
+    } catch (_) {}
+  }
+
+  function notifyParentFrameReady() {
+    if (!isHubLinked()) return;
+    try {
+      window.parent.postMessage({ type: "instance-frame-ready" }, "*");
+    } catch (_) {}
+  }
+
+  /** 中控 iframe 内：拦截顶栏导航，fetch 后 document.write 原地换页，避免 iframe 卸载白屏 */
+  function initHubEmbedInFrameNav() {
+    if (!isHubLinked()) return;
+
+    let navToken = 0;
+
+    function isSoftNavLink(a) {
+      if (!a || !a.getAttribute) return false;
+      return !!a.closest(".top-nav, .strategy-subnav");
+    }
+
+    async function navigateInFrame(href, opts) {
+      const token = ++navToken;
+      notifyParentFrameNavStart();
+      try {
+        const r = await fetch(href, { credentials: "same-origin" });
+        if (token !== navToken) return;
+        if (!r.ok) {
+          location.href = href;
+          return;
+        }
+        const html = await r.text();
+        if (token !== navToken) return;
+        document.open();
+        document.write(html);
+        document.close();
+        let path = href;
+        try {
+          const u = new URL(href, location.href);
+          path = u.pathname + u.search + u.hash;
+        } catch (_) {}
+        if (opts && opts.replace) history.replaceState(null, "", path);
+        else history.pushState(null, "", path);
+      } catch (_) {
+        if (token === navToken) location.href = href;
+      }
+    }
+
+    document.addEventListener(
+      "click",
+      (ev) => {
+        const a = ev.target.closest("a[href]");
+        if (!a || !isSoftNavLink(a) || ev.defaultPrevented) return;
+        if (ev.button !== 0 || ev.ctrlKey || ev.metaKey || ev.shiftKey || ev.altKey) return;
+        const rawHref = a.getAttribute("href");
+        if (!rawHref || rawHref.startsWith("#") || rawHref.startsWith("javascript:")) return;
+        let target;
+        try {
+          target = new URL(rawHref, location.href);
+        } catch (_) {
+          return;
+        }
+        if (target.origin !== location.origin) return;
+        const nextHref = target.pathname + target.search + target.hash;
+        if (target.pathname === location.pathname && target.search === location.search) return;
+        ev.preventDefault();
+        void navigateInFrame(nextHref);
+      },
+      true
+    );
+
+    window.addEventListener("popstate", () => {
+      void navigateInFrame(location.pathname + location.search + location.hash, { replace: true });
+    });
+  }
+
   function boot() {
     if (isHubLinked()) {
       apply(get(), { skipStore: true });
       window.addEventListener("message", (ev) => initFromHubMessage(ev.data));
+      initHubEmbedInFrameNav();
+      notifyParentFrameReady();
       try {
         window.parent.postMessage({ type: "instance-theme-ready" }, "*");
       } catch (_) {}

static/instance_theme_early.css

@@ -1,4 +1,14 @@
 /* 紧接 instance_theme.js 之后加载，避免亮色下先闪暗色底 */
+html {
+  background: #0b0d14;
+  color-scheme: dark;
+}
+
+html[data-theme="light"] {
+  background: #d8e2ec;
+  color-scheme: light;
+}
+
 html[data-theme="light"] body {
   background: #d8e2ec !important;
   color: #1a2838 !important;