fix(sync): stop a local sync reporting itself as a background one

Reported from hand testing: a manual sync ended with "Sync finished
elsewhere" stamped over its own result.

Ownership of a lock period was being decided when the lock was RELEASED,
by asking MailSync::isRunning(). That question cannot be answered then:
the process exits, so isRunning() goes false, and only afterwards does
the next poll observe the lock gone. The guard therefore suppressed the
message while the sync ran and let it through at the end, up to two
seconds after onSyncFinished() had already said what happened.

Ownership is now latched when the lock APPEARS, which is the moment
isRunning() can still answer, and the matching release is swallowed.
onSyncFinished() hands the latch back when it sees exit 75, because a
skip means the lock was never ours: if a manual run and the cron run
start inside one poll interval, the lock would otherwise be latched as
local and that other run's completion swallowed with it.

Also renames the messages to "Background sync running/completed" per the
user: "finished elsewhere" reads as though the application does not know
what is syncing the Maildir, when in fact it is the same script.

The tests added here cover the external path and the Unknown state. They
do NOT reproduce the reported bug, and were checked against a reverted
fix to confirm that: staging it needs isRunning() true at the Running
transition and false at the Idle one, which cannot be arranged in
test_mainwindow without a configured sync command and a live child
process. That was tried and abandoned, it left a process running for the
length of the suite and popped a dialog. The ordering and the fix were
instead verified against a standalone model of both code paths.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/mainwindow.h b/src/mainwindow.h
index 57aceb1..66044dc 100644
--- a/src/mainwindow.h
+++ b/src/mainwindow.h
@@ -115,6 +115,12 @@ private slots:
     void onWorkerError(const QString &message);
     void onSyncFinished(bool success, int exitCode);
 
+    /// Reacts to a sync started outside this window, by cron or by hand.
+    ///
+    /// A private slot rather than a plain method so tests can drive it through
+    /// the meta-object without widening the public API.
+    void onExternalSyncStateChanged(SyncMonitor::State state);
+
     /// A tag mutation the worker has confirmed reached the database. Counts it
     /// as unsynced, since reaching the index is not reaching the mail store.
     void onTagsApplied(const TagChange &change);
@@ -169,8 +175,6 @@ private:
     /// says "working, duration unknown", which is the truth.
     void setSyncBusy(bool busy);
 
-    /// Reacts to a sync started outside this window, by cron or by hand.
-    void onExternalSyncStateChanged(SyncMonitor::State state);
 
     /// Opens the tag dialog on the current selection and applies its result.
     ///
@@ -213,6 +217,12 @@ private:
 
     /// Watches the sync lock for runs this window did not start.
     SyncMonitor *m_syncMonitor = nullptr;
+
+    /// True while the lock the monitor can see is held by this window's own
+    /// sync. Latched when the lock is taken, because by the time it is released
+    /// MailSync::isRunning() is already false and can no longer answer "was
+    /// that ours?".
+    bool m_localSyncHoldsLock = false;
     QUndoStack m_undoStack;
 
     QLineEdit *m_queryEdit = nullptr;