fix(recovery): verify offline browser state before banner (#4170)

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **Offline recovery now verifies unreliable browser offline reports with the existing `/health` probe before showing or sticking on the browser-reason banner (#4170).** `navigator.onLine` is treated as a hint for the banner reason, while server reachability remains the source of truth, so browsers that incorrectly report offline can recover without a page reload or permanent connection-lost state. (#4170)
+
 ## [v0.51.410] — 2026-06-14 — Release NW (chat Mermaid lightbox + workspace CSV table preview, #4075/#4025)
 
 ### Added

static/ui.js

@@ -85,16 +85,24 @@ async function _probeOfflineRecovery(){
   try{return await _offlineHealthProbePromise;}
   finally{_offlineHealthProbePromise=null;}
 }
+async function _showOfflineBannerIfProbeFails(reason){
+  const ok=await _probeOfflineRecovery();
+  if(ok){
+    if(_offlineVisible){_stopOfflineProbeTimer();await _recoverFromOfflineSoftly();}
+    return true;
+  }
+  showOfflineBanner(reason||(_browserReportsOnline()?'network':'browser'));
+  return false;
+}
 async function checkOfflineRecoveryNow(){
   if(_offlineProbePromise)return _offlineProbePromise;
   _offlineProbePromise=(async()=>{
     if(!_offlineVisible)return false;
-    if(!_browserReportsOnline()){showOfflineBanner('browser');return false;}
     _setOfflineChecking(true);
     const ok=await _probeOfflineRecovery();
     _setOfflineChecking(false);
     if(ok){_stopOfflineProbeTimer();await _recoverFromOfflineSoftly();return true;}
-    showOfflineBanner('network');
+    showOfflineBanner(_browserReportsOnline()?'network':'browser');
     return false;
   })();
   try{return await _offlineProbePromise;}
@@ -153,17 +161,18 @@ function _patchOfflineFetch(){
   window.fetch=async function(...args){
     try{return await _offlineRawFetch(...args);}
     catch(e){
-      if(!_browserReportsOnline())showOfflineBanner('browser');
-      else if(e instanceof TypeError&&!_isAbortError(e))void _probeOfflineRecovery().then(ok=>{if(!ok)showOfflineBanner('network');});
+      if(!_isAbortError(e)&&(e instanceof TypeError||!_browserReportsOnline())){
+        void _showOfflineBannerIfProbeFails(_browserReportsOnline()?'network':'browser');
+      }
       throw e;
     }
   };
 }
 function initOfflineMonitor(){
   _patchOfflineFetch();
-  window.addEventListener('offline',()=>showOfflineBanner('browser'));
+  window.addEventListener('offline',()=>{void _showOfflineBannerIfProbeFails('browser');});
   window.addEventListener('online',()=>{if(_offlineVisible)checkOfflineRecoveryNow();});
-  if(!_browserReportsOnline())showOfflineBanner('browser');
+  if(!_browserReportsOnline())void _showOfflineBannerIfProbeFails('browser');
 }
 if(document.readyState==='loading')document.addEventListener('DOMContentLoaded',initOfflineMonitor,{once:true});
 else initOfflineMonitor();

tests/test_4170_offline_probe.py

@@ -0,0 +1,65 @@
+"""Regression coverage for unreliable navigator.onLine offline reports."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+UI_JS = (ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _strip_comments(src: str) -> str:
+    src = re.sub(r"/\*.*?\*/", "", src, flags=re.S)
+    return re.sub(r"//.*", "", src)
+
+
+def _fn_body(src: str, marker: str) -> str:
+    idx = src.find(marker)
+    assert idx != -1, f"{marker!r} not found"
+    brace = src.find("{", idx)
+    assert brace != -1, f"{marker!r} has no body"
+    depth = 1
+    i = brace + 1
+    while i < len(src) and depth:
+        if src[i] == "{":
+            depth += 1
+        elif src[i] == "}":
+            depth -= 1
+        i += 1
+    assert depth == 0, f"{marker!r} body did not close"
+    return src[brace + 1 : i - 1]
+
+
+def test_recovery_loop_probes_even_when_browser_reports_offline():
+    body = _strip_comments(_fn_body(UI_JS, "async function checkOfflineRecoveryNow("))
+    assert "_probeOfflineRecovery()" in body
+    assert "if(!_browserReportsOnline()){showOfflineBanner('browser');return false;}" not in body
+    assert body.find("_probeOfflineRecovery()") < body.find("showOfflineBanner(")
+    assert "showOfflineBanner(_browserReportsOnline()?'network':'browser')" in body
+
+
+def test_probe_backed_browser_banner_helper_uses_health_as_source_of_truth():
+    body = _strip_comments(_fn_body(UI_JS, "async function _showOfflineBannerIfProbeFails("))
+    assert "const ok=await _probeOfflineRecovery();" in body
+    assert "if(ok)" in body
+    assert "showOfflineBanner(reason||(_browserReportsOnline()?'network':'browser'))" in body
+    assert body.find("_probeOfflineRecovery()") < body.find("showOfflineBanner(")
+
+
+def test_offline_event_and_startup_false_online_probe_before_browser_banner():
+    body = _strip_comments(_fn_body(UI_JS, "function initOfflineMonitor("))
+    assert "window.addEventListener('offline',()=>showOfflineBanner('browser'))" not in body
+    assert "if(!_browserReportsOnline())showOfflineBanner('browser');" not in body
+    assert "window.addEventListener('offline',()=>{void _showOfflineBannerIfProbeFails('browser');});" in body
+    assert "if(!_browserReportsOnline())void _showOfflineBannerIfProbeFails('browser');" in body
+
+
+def test_fetch_catch_probes_before_browser_banner_and_rethrows():
+    body = _strip_comments(_fn_body(UI_JS, "function _patchOfflineFetch("))
+    fetch_body = body.split("window.fetch=async function(...args){", 1)[1]
+    assert "if(!_browserReportsOnline())showOfflineBanner('browser');" not in fetch_body
+    assert "void _showOfflineBannerIfProbeFails(_browserReportsOnline()?'network':'browser');" in fetch_body
+    assert "throw e;" in fetch_body
+    assert fetch_body.find("_showOfflineBannerIfProbeFails(") < fetch_body.find("throw e;")