fix(collab): swap the participant sidebar when the iframe switches sessions

packages/app/src/pages/collab/session.tsx

@@ -547,6 +547,35 @@ function CollabSessionInner(props: { me: Me }) {
         void collab.setTyping(Boolean(data.typing))
         return
       }
+
+      // Native session switched inside the iframe (e.g. opencode's
+      // cross-session "go to session" popup).  Swap the whole collab page —
+      // including the participant sidebar — to the collab session that owns
+      // that native session, so the left panel matches the conversation shown
+      // on the right.
+      if (data.type === "opencode:collab-session-changed") {
+        const nativeId = typeof data.sessionId === "string" ? data.sessionId : ""
+        if (!nativeId) return
+        // Already showing this native session → nothing to do (also breaks the
+        // post-navigation echo: the freshly-loaded iframe re-announces its id).
+        if (nativeId === collab.session()?.sessionId) return
+        void (async () => {
+          try {
+            const res = await fetch("/collab/session")
+            if (!res.ok) return
+            const list = (await res.json()) as Array<{ id: string; sessionId?: string | null }>
+            const target = list.find((s) => s.sessionId === nativeId)
+            if (target && target.id !== collab.session()?.id) {
+              // Full page nav so CollabProvider remounts with the new session
+              // (fresh participants + SSE) — same pattern as the Collab pill.
+              window.location.href = `/collab/${target.id}`
+            }
+          } catch {
+            /* best-effort — leave the page as-is if the lookup fails */
+          }
+        })()
+        return
+      }
     }
     window.addEventListener("message", onIframeMessage)
     onCleanup(() => window.removeEventListener("message", onIframeMessage))

packages/app/src/pages/layout.tsx

@@ -112,6 +112,30 @@ export default function Layout(props: ParentProps) {
   let dialogDead = false
 
   const params = useParams()
+
+  // Collab embed: when the iframe's active native session changes — e.g. via
+  // opencode's lower-right "go to session" cross-session popup — tell the
+  // parent collab page so it can swap to the matching collab session and
+  // refresh the participant sidebar.  `params.id` is the native session id;
+  // `defer` skips the initial mount (the parent is already on the right
+  // session then) and prevents an echo loop after the parent navigates.
+  if (isCollabEmbed()) {
+    createEffect(
+      on(
+        () => params.id,
+        (sessionId) => {
+          if (!sessionId) return
+          try {
+            window.parent.postMessage({ type: "opencode:collab-session-changed", sessionId }, window.location.origin)
+          } catch {
+            /* same-origin postMessage shouldn't throw; ignore if it does */
+          }
+        },
+        { defer: true },
+      ),
+    )
+  }
+
   const globalSDK = useGlobalSDK()
   const globalSync = useGlobalSync()
   const layout = useLayout()