Harden dm.py sidechat navigation (CDP event drain + Page.navigate)

- ev(): drain CDP events until response id==1 (was blind recv,
  same bug class as box-chat-cdp.py 8d4bfa7 NO_SWITCHER fix)
- cmd_sidechat_use UUID path: use CDP Page.navigate instead of
  window.location.href via evaluate; 5 attempts, 15s SPA settle
  per attempt (was 3x4s, flaked 1/3 on url_mismatch)
- cmd_sidechat_use name path: retry 3x if not landing on /thread/

Test: 2/2 DM sends succeeded when browser healthy (tests 3-5
hit 646 browser death mid-test, infra issue not nav issue).

Session: sidechat/chromebox-ops

bin/muse-chat-api.py

@@ -69,7 +69,18 @@ def ev(ws, expr, await_p=False):
         "id": 1, "method": "Runtime.evaluate",
         "params": {"expression": expr, "returnByValue": True, "awaitPromise": await_p}
     }))
+    # Drain CDP events until we get our command response (id 1).
+    # The browser can emit events (Runtime.executionContextCreated, etc.)
+    # at any time; taking the first recv() blindly returns None on a
+    # busy page (observed as transient navigation failures in dm.py
+    # sidechat sends, 2026-10-04 — same class as the NO_SWITCHER fix
+    # in box-chat-cdp.py commit 8d4bfa7).
+    for _ in range(50):
         resp = json.loads(ws.recv())
+        if resp.get("id") == 1:
+            break
+    else:
+        return None
     return resp.get('result', {}).get('result', {}).get('value')
 
 def check_approvals(ws):
@@ -215,6 +226,27 @@ def cmd_wait(ws, timeout=30):
         time.sleep(5)
     cmd_messages(ws, 2)
 
+def cdp_navigate(ws, url, timeout_s=30):
+    """Navigate via CDP Page.navigate (proper navigation, waits for commit).
+    Returns True if the page URL matches the target after navigation."""
+    import time as _time
+    ws.send(json.dumps({"id": 2, "method": "Page.navigate",
+                        "params": {"url": url}}))
+    # Drain until we get the Page.navigate response (id 2).
+    for _ in range(50):
+        resp = json.loads(ws.recv())
+        if resp.get("id") == 2:
+            break
+    else:
+        return False
+    # Wait for the URL to settle (SPA client-side routing).
+    for _ in range(timeout_s):
+        cur = ev1(ws, "window.location.href", True)
+        if cur and url.rstrip("/").lower() in cur.lower():
+            return True
+        _time.sleep(1)
+    return False
+
 def cmd_sidechat_use(ws, chat_id):
     """Open a sidechat by name (sidebar text search) or by thread UUID
     (direct navigation). The sidebar shows titles, not UUIDs, so the
@@ -224,15 +256,19 @@ def cmd_sidechat_use(ws, chat_id):
     if re.fullmatch(r"[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}", cid.lower()):
         url = "https://muse.ai/thread/" + cid.lower()
         want_uuid = cid.lower()
-        # Use ev1 (skips CDP chatter) and confirm the URL actually changed.
-        # A stale read here used to report the previous thread's URL (2026-10-04).
+        # Use CDP Page.navigate (proper navigation lifecycle) and confirm
+        # the URL actually changed. Hardened 2026-10-04: the React SPA
+        # sometimes redirects back to / when the thread page hasn't finished
+        # loading; retry instead of failing immediately.
+        # (dm.py sidechat 1/3 flake; was window.location.href via evaluate.)
         cur = None
-        for _try in range(3):
-            ev1(ws, "window.location.href=" + json.dumps(url), True)
-            time.sleep(4)
+        for _try in range(5):
+            ok = cdp_navigate(ws, url, timeout_s=15)
             cur = ev1(ws, "window.location.href", True)
-            if cur and want_uuid in cur:
+            if ok and cur and want_uuid in cur:
                 break
+            # SPA dropped the nav or hasn't routed yet; wait and retry.
+            time.sleep(3)
         print(f"Navigated to: {cur}")
         return
     b64 = base64.b64encode(chat_id.encode()).decode()
@@ -255,7 +291,15 @@ def cmd_sidechat_use(ws, chat_id):
         "return window.location.href;"
         "})()"
     )
+    # Hardened 2026-10-04: the click sometimes doesn't navigate (React
+    # mid-render, or the SPA drops it). Retry the whole nav if we didn't
+    # land on a /thread/ URL.
+    result = None
+    for _try in range(3):
         result = ev(ws, js, True)
+        if result and "/thread/" in result:
+            break
+        time.sleep(3)
     print(f"Navigated to: {result}")
 
 def cmd_sidechat_main(ws):