Microsoft Edge Sync -- "Setting up sync" never completes, and sync never starts
after a cold start. Only a fresh sign-in restores sync, for exactly one session.

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ENVIRONMENT
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Edge version      : 156.0.4314.42 (stable channel, also reproduced on 155.0.4283.62)
OS                : Windows 10 Home 26H2, build 26300.9711, AMD64
Account type      : MSA (personal Microsoft account)
Network           : two independent networks tested
                    - home broadband (China Unicom)
                    - USB phone tethering (China Telecom)
Proxy             : none (system proxy disabled); also tested with a local proxy
Policy            : no Edge policies configured (HKLM/HKCU Policies\Microsoft\Edge absent)
Profile           : reproduced on 3 separate profiles (2 freshly created)
Extensions        : 4 installed

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STEPS TO REPRODUCE
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1. Sign out of Edge sync, then sign in again (or create a new profile and sign in).
   -> Sync works. Transport State becomes Active, data downloads, commits succeed.
2. Leave Edge running; sync keeps working.
3. Fully quit Microsoft Edge (all msedge.exe processes terminated).
4. Launch Microsoft Edge again normally (Start menu / taskbar shortcut, no flags).
5. Open edge://sync-internals.

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EXPECTED
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Sync resumes automatically. Transport State = Active, data types Running.

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ACTUAL
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Sync never starts and never retries. The UI shows "Setting up sync" ("姝ｅ湪璁剧疆鍚屾")
indefinitely. Values observed in edge://sync-internals after the restart:

  Transport State            : Initializing
  Server Connection          : not attempted
  Server Environment         : no_env-v1
  Sync Engine                : Backend Status: Initialized; BlockReason: NotStarted; Syncer: Unset
  DataType Manager           : State: Stopped
  MSA Account Key Status     : Download Key Result: Fail
                               Net Error Code: n/a
                               HTTP Response Code: n/a
  All data types             : Not Running
  Running Totals             : Updates Downloaded 0 / Successful Commits 0
  User Actionable Error      : None

Note that "Net Error Code" and "HTTP Response Code" are both "n/a", i.e. no network
request is ever attempted. This is not a connectivity failure: with the same machine,
same network, DNS resolution for edge.microsoft.com is correct, TCP 443 to
edge.microsoft.com / login.microsoftonline.com / login.live.com all succeed, and the
Windows connectivity probe (www.msftconnecttest.com/connecttest.txt) returns
"Microsoft Connect Test".

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EVIDENCE FROM SYNC ENGINE LOG
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File: <User Data>\<Profile>\Sync Data\Logs\sync_diagnostic.log

Healthy start (first session after sign-in) -- note the control type is registered:

  [INFO][SyncEngineBackend::DoInitialize] Control Types added: Encryption Keys
  [INFO][SyncEngineBackend::DoFinishConfigureDataTypes] ... succeeded types: Passwords, Device Info, Encryption Keys
  [INFO][Sync]     Configuration completed, state: 3
  [INFO][Sync] Configured DataTypeManager: Ok
  [INFO][SyncEngineBackend::DoStartSyncing] Start syncing

Broken start (every cold start) -- note the control type list is EMPTY:

  [INFO][Sync] SyncState after authenticated was: FeatureCanStart
  [INFO][Sync] Try to start sync engine
  [INFO][SyncEngineBackend::LoadAndConnectNigoriController] Load and connect Nigori controller
  [INFO][SyncEngineBackend::DoInitialize] Control Types added:
  [INFO][SyncManagerImpl::ConfigureSyncer] Types to download:  with reason: 4
  [INFO][SyncEngineBackend::DoUpdateKeyDataCredentials] Update key data credentials
  [INFO][SyncAuthManager::SetLastAuthError] Current auth error: None
  [INFO][Sync] Credentials changed for: EdgeSyncScopeNew
  [INFO][SyncEngineBackend::DoUpdateCredentials] Update credentials
  <-------- and then nothing at all. No ConfigureSyncer with a real type list,
            no Start syncing, and no further log lines ever.

The corresponding HTTP traffic log (cv_debug.log) is not written to at all after the
restart -- in one case it stayed untouched for 6 days while the browser was used daily,
proving that zero sync requests were issued.

Authentication is NOT the problem: the token requests succeed at every start
("Last Token Request Result: OK", "Current auth error: None"), Edge sign-in telemetry
reports "authoutcome: succeeded" 19/19 times, and the same account syncs successfully
immediately after a fresh sign-in.

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WHAT HAS BEEN RULED OUT
===============================================================
[x] Network / connectivity / DNS / captive portal -- two different networks, all
    Microsoft endpoints reachable, no captive portal.
[x] Proxy / VPN -- reproduced with no proxy at all.
[x] Account / credentials / MFA -- token flow is OK at every start; fresh sign-in
    works immediately with the same account.
[x] Edge policies / flags -- no policies configured, no experimental flags.
[x] Corrupted local sync store -- deleted the whole "Sync Data" directory
    (renamed to Sync Data.bak) while signed out, signed in again (sync rebuilt and
    worked), then cold started: exactly the same empty "Control Types added:" and
    zero requests.
[x] Profile corruption -- reproduced on 3 profiles, 2 of them created from scratch.
[x] Version-specific -- reproduced on both 155.0.4283.62 and 156.0.4314.42.
[x] Startup boost / continue-running-in-background -- disabled, still reproduces.

===============================================================
ADDITIONAL OBSERVATIONS
===============================================================
1. In the healthy session, Edge sync traffic goes to
   https://edge.microsoft.com/sync/v1/feeds/me/syncEntities
   and responses report environment "Prod_japaneast_prod-s01-061-apac-japaneast".
   In the broken state, Server Environment shows "no_env-v1", which appears to be the
   placeholder value used before any server response is received.

2. In Local State there is a key:
       "initial_sync_session_startup_pref_state": 1
   Its meaning is not documented; flagging it in case it is related.

3. Killing all msedge.exe processes makes Edge relaunch itself repeatedly (6 rounds of
   kill were required), so "fully closing the browser" is non-trivial on this machine.

4. Workaround that does work, temporarily:
       edge://settings/profiles -> sign out (keep local data) -> sign in
   Sync then works for the rest of that browser session only.

===============================================================
IMPACT
===============================================================
Sync is effectively unusable on this machine: any browser restart or reboot leaves
sync permanently stalled with no error surfaced to the user, and the only recovery is
a manual sign-out/sign-in after every restart.

Please investigate why the sync engine is initialized with an empty control/data type
set on cold start, and why it does not retry or surface an error in that state.