Improve crash diagnostics and isolate watcher-thread failures

Main-thread unhandled exceptions now show the traceback and log path; worker-thread exceptions log without touching Qt widgets. The download watcher catches its own loop failures and reports that it stopped instead of surfacing a blank error dialog. Adds a regression test for the watcher failure path.

Co-Authored-By: Claude Code <noreply@anthropic.com>

build_windows.ps1

@@ -10,15 +10,19 @@ $Root = Split-Path -Parent $MyInvocation.MyCommand.Path
 Set-Location $Root
 
 function Get-Python {
-    $candidates = @("py -3.11", "py -3", "python")
+    $candidates = @(
+        @{ Command = "py"; Arguments = @("-3.11") },
+        @{ Command = "py"; Arguments = @("-3") },
+        @{ Command = "python"; Arguments = @() },
+        @{ Command = "python3"; Arguments = @() }
+    )
     foreach ($candidate in $candidates) {
         try {
-            $parts = $candidate.Split(" ")
-            $exe = $parts[0]
-            $exeArgs = @($parts | Select-Object -Skip 1)
-            $output = & $exe @exeArgs -c "import sys; print(sys.executable); print(sys.version_info[:2])" 2>$null
-            if ($LASTEXITCODE -eq 0 -and $output -match "3\.11") {
-                return $parts
+            $exe = $candidate.Command
+            $exeArgs = @($candidate.Arguments)
+            $probe = & $exe @exeArgs -c "import sys; print('{0}.{1}'.format(sys.version_info[0], sys.version_info[1]))" 2>$null
+            if ($LASTEXITCODE -eq 0 -and (($probe | Out-String).Trim() -eq "3.11")) {
+                return $candidate
             }
         } catch { }
     }
@@ -31,8 +35,8 @@ $python = Get-Python
 
 if (-not (Test-Path $venv)) {
     Write-Host "Creating build environment..."
-    $pythonExe = $python[0]
-    $pythonArgs = @($python | Select-Object -Skip 1)
+    $pythonExe = $python.Command
+    $pythonArgs = @($python.Arguments)
     & $pythonExe @pythonArgs -m venv $venv
 }
 $py = Join-Path $venv "Scripts\python.exe"

src/boltclam/app.py

@@ -5,7 +5,9 @@ from __future__ import annotations
 import logging
 import os
 import sys
+import threading
 import time
+import traceback
 from pathlib import Path
 
 from PyQt6.QtCore import QEventLoop, QLockFile, QRectF, Qt, QTimer
@@ -18,7 +20,7 @@ from .auth import AuthService, CredentialStore, Session, SessionExpired
 from .clamav import ClamAVService
 from .config import AccountCache, SettingsStore
 from .context import AppContext
-from .logging_setup import configure as configure_logging
+from .logging_setup import configure as configure_logging, log_path
 from .quarantine import QuarantineService
 from .storage import Storage
 from .ui import brand, theme
@@ -50,20 +52,19 @@ def _splash_pixmap() -> QPixmap:
     brand.paint_mark(painter, QRectF(34, 34, 72, 72))
     painter.setPen(QColor(theme.TEXT))
     painter.setFont(theme.display_font(26, QFont.Weight.DemiBold))
-    painter.drawText(QRectF(124, 36, 300, 40), Qt.AlignmentFlag.AlignLeft,
-                     Qt.AlignmentFlag.AlignVCenter, "BoltClam")
+    left_center = int(Qt.AlignmentFlag.AlignLeft | Qt.AlignmentFlag.AlignVCenter)
+    painter.drawText(QRectF(124, 36, 300, 40), left_center, "BoltClam")
     painter.setPen(QColor(theme.MUTED))
     painter.setFont(theme.body_font(10.0))
-    painter.drawText(QRectF(126, 76, 300, 24), Qt.AlignmentFlag.AlignLeft,
-                     Qt.AlignmentFlag.AlignVCenter, "ClamAV protection, without the noise.")
+    painter.drawText(QRectF(126, 76, 300, 24), left_center,
+                     "ClamAV protection, without the noise.")
     painter.setPen(QColor(theme.TEXT_SOFT))
     painter.setFont(theme.body_font(10.5))
-    painter.drawText(QRectF(36, 154, 390, 26), Qt.AlignmentFlag.AlignLeft,
-                     Qt.AlignmentFlag.AlignVCenter, "Starting protection…")
+    painter.drawText(QRectF(36, 154, 390, 26), left_center, "Starting protection…")
     painter.setPen(QColor(theme.DIM))
     painter.setFont(theme.mono_font(8.0))
-    painter.drawText(QRectF(36, 206, 390, 20), Qt.AlignmentFlag.AlignLeft,
-                     Qt.AlignmentFlag.AlignVCenter, f"Version {__version__} · {__build_date__}")
+    painter.drawText(QRectF(36, 206, 390, 20), left_center,
+                     f"Version {__version__} · {__build_date__}")
     painter.end()
     return image
 
@@ -74,10 +75,17 @@ def _install_exception_hook() -> None:
             sys.__excepthook__(exc_type, exc_value, exc_traceback)
             return
         logger.critical("Unhandled exception", exc_info=(exc_type, exc_value, exc_traceback))
+        # Qt widget calls are not safe from worker threads. Log there and
+        # leave the UI alone; the page's own status/toast reports the failure.
+        if threading.current_thread() is not threading.main_thread():
+            return
+        detail = "".join(traceback.format_exception(exc_type, exc_value, exc_traceback)).strip()
+        if len(detail) > 3200:
+            detail = "…\n" + detail[-3200:]
         try:
             QMessageBox.critical(
                 None, "BoltClam encountered an error",
-                f"{exc_type.__name__}: {exc_value}\n\nDetails were written to the BoltClam log.")
+                f"{detail}\n\nFull details: {log_path()}")
         except Exception:
             pass
 

src/boltclam/realtime.py

@@ -259,6 +259,16 @@ class DownloadWatcher(QThread):
             self._stop.set()
 
     def run(self) -> None:
+        try:
+            self._watch_loop()
+        except Exception:
+            logger.exception("Real-time watcher stopped after an error")
+            try:
+                self.watch_status.emit("Real-time watcher stopped")
+            except Exception:
+                pass
+
+    def _watch_loop(self) -> None:
         import threading
 
         stop = threading.Event()

src/boltclam/ui/components.py

@@ -142,7 +142,7 @@ class Avatar(QWidget):
         painter.setPen(QColor(theme.AMBER_BRIGHT))
         font = theme.display_font(self._size * 0.30, QFont.Weight.DemiBold)
         painter.setFont(font)
-        painter.drawText(rect, Qt.AlignmentFlag.AlignCenter, self.initials())
+        painter.drawText(rect, int(Qt.AlignmentFlag.AlignCenter), self.initials())
 
 
 class ProtectionDial(QWidget):
@@ -331,9 +331,10 @@ class NavButton(QAbstractButton):
         painter.setPen(QColor(theme.TEXT if checked else theme.TEXT_SOFT))
         font = theme.body_font(10.4, QFont.Weight.DemiBold if checked else QFont.Weight.Normal)
         painter.setFont(font)
-        painter.drawText(QRectF(46, 0, self.width() - 52, self.height()),
-                         Qt.AlignmentFlag.AlignVCenter | Qt.AlignmentFlag.AlignLeft,
-                         self.text())
+        painter.drawText(
+            QRectF(46, 0, self.width() - 52, self.height()),
+            int(Qt.AlignmentFlag.AlignVCenter | Qt.AlignmentFlag.AlignLeft),
+            self.text())
 
 
 def _draw_nav_icon(painter: QPainter, name: str, rect: QRectF, color: QColor) -> None:
@@ -529,9 +530,10 @@ class ScanButton(QAbstractButton):
 
         painter.setPen(QColor("#241704" if self.isEnabled() else "#807A68"))
         painter.setFont(theme.body_font(11.2, QFont.Weight.Bold))
-        painter.drawText(QRectF(core.left(), core.center().y() + 20, core.width(), 24),
-                         Qt.AlignmentFlag.AlignCenter,
-                         "Scanning…" if self._scanning else "Start scan")
+        painter.drawText(
+            QRectF(core.left(), core.center().y() + 20, core.width(), 24),
+            int(Qt.AlignmentFlag.AlignCenter),
+            "Scanning…" if self._scanning else "Start scan")
 
 
 class StatTile(QFrame):

tests/test_realtime.py

@@ -58,6 +58,19 @@ def test_firefox_download_preference_is_watched(tmp_path, monkeypatch):
     assert download.resolve() in [path.resolve() for path in roots]
 
 
+def test_watcher_stops_cleanly_instead_of_raising(monkeypatch, qapp):
+    messages: list[str] = []
+    watcher = realtime.DownloadWatcher(interval=0.1)
+    watcher.watch_status.connect(messages.append)
+    monkeypatch.setattr(realtime, "browser_download_roots", lambda extra: (_ for _ in ()).throw(
+        RuntimeError("simulated watcher failure")))
+    watcher.start()
+    assert watcher.wait(3000)
+    qapp.processEvents()
+    assert not watcher.isRunning()
+    assert "stopped" in " ".join(messages).lower()
+
+
 def test_copy_for_scan_preserves_bytes(tmp_path):
     source = tmp_path / "download.exe"
     source.write_bytes(b"sample bytes")

tests/test_ui_smoke.py

@@ -66,12 +66,24 @@ def test_main_window_builds_four_pages_and_switches_tabs(qapp, isolated_env):
         assert window.stack.currentIndex() == 3
         assert window.settings_page.channel_combo.currentData() == "release"
         assert window.about_page.rows["account"].value_label.text() == "Aaron"
+        for index in range(4):
+            window.stack.setCurrentIndex(index)
+            qapp.processEvents()
+            assert not window.stack.currentWidget().grab().isNull()
     finally:
         window._allow_close = True
         window.close()
         qapp.processEvents()
 
 
+def test_splash_pixmap_renders(qapp):
+    from boltclam.app import _splash_pixmap
+
+    pixmap = _splash_pixmap()
+    assert not pixmap.isNull()
+    assert pixmap.width() == 460 and pixmap.height() == 250
+
+
 def test_login_dialog_builds_without_starting_a_worker(qapp, isolated_env):
     auth = AuthService(
         "http://127.0.0.1:9", install_id="test",