fix: keep bulk tag actions linear, and say when they land

Select all on a 10k-row search, then Archive, froze the window with one
CPU pinned. gdb on the live process showed the UI thread cubic before the
write was ever sent: each optimistic row write emitted dataChanged, whose
handler refreshed the action labels by walking the whole selection, and
each step of that walk looked the message up with a linear scan of every
thread.

ThreadListModel now answers id lookups from a lazily rebuilt hash,
invalidated by its own structural signals and by hand at the two in-place
id changes. MainWindow::batchModelWrites() holds the label refresh while a
loop of optimistic writes runs, then refreshes once. Two list membership
tests became sets.

The announcement also used to fade on a timer whether or not the write
had landed. It now reads "applying" until a round trip through the
worker's queue says the write has run, then "done". An error takes the
label back from a late "done".

Item 211.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 86 insertions, 0 deletions

diff --git a/tests/test_mainwindow.cpp b/tests/test_mainwindow.cpp
index 2b7008d..71795fa 100644
--- a/tests/test_mainwindow.cpp
+++ b/tests/test_mainwindow.cpp
@@ -295,6 +295,7 @@ private slots:
     void narrowingAnEmptyQueryBarIsAPlainSearch();
     void aMalformedAccountIsReportedWithoutBlockingTheConstructor();
     void aWorkerBackedWindowReturnsRealThreads();
+    void aWriteSaysItIsApplyingUntilTheWorkerHasRunIt();
     void aPurgeTakesTheRowsOutOfTheViewWithoutARefresh();
     void theDashboardFollowsAWriteToTheConversationItShows();
     void anEditHeldByASyncSaysSoInsteadOfClaimingItLanded();
@@ -9892,6 +9893,42 @@ void TestMainWindow::aWorkerBackedWindowReturnsRealThreads()
     QTRY_VERIFY_WITH_TIMEOUT(model->rowCount(QModelIndex()) == 1, 15000);
 }
 
+void TestMainWindow::aWriteSaysItIsApplyingUntilTheWorkerHasRunIt()
+{
+    // Item 211, from the user: after archiving ten thousand rows, "how do I
+    // know if it worked? The UI doesn't give me a feedback". The announcement
+    // used to vanish on a timer whether or not the write had landed.
+    WorkerBackedWindow backed;
+    QVERIFY(backed.fixture().addMessage(
+        QStringLiteral("inbox"), QStringLiteral("root@example.org"),
+        QStringLiteral("A subject"), QStringLiteral("sender@example.org"),
+        // Friday, verified with `date -d 2026-08-14 +%A`.
+        QStringLiteral("Fri, 14 Aug 2026 10:00:00 +0200"),
+        QStringLiteral("Body text.")));
+    QVERIFY2(backed.build(), qPrintable(backed.error()));
+
+    MainWindow window(backed.config());
+    auto *queryEdit = window.findChild<QLineEdit *>(QStringLiteral("queryEdit"));
+    auto *model = window.findChild<ThreadListModel *>();
+    auto *view = window.findChild<ThreadListView *>();
+    auto *status = window.findChild<QLabel *>(QStringLiteral("statusMessage"));
+    QVERIFY(queryEdit && model && view && status);
+
+    queryEdit->setText(QStringLiteral("tag:inbox"));
+    queryEdit->returnPressed();
+    QTRY_VERIFY_WITH_TIMEOUT(model->rowCount(QModelIndex()) == 1, 15000);
+    view->setCurrentIndex(model->index(0, 0, QModelIndex()));
+
+    window.findChild<QAction *>(QStringLiteral("archive"))->trigger();
+
+    // Synchronous: the worker's reply is posted to this thread and cannot be
+    // delivered before the event loop runs, so this cannot race.
+    QVERIFY2(status->text().contains(QStringLiteral("applying")),
+             qPrintable(status->text()));
+    QTRY_VERIFY2_WITH_TIMEOUT(status->text().contains(QStringLiteral("done")),
+                              qPrintable(status->text()), 15000);
+}
+
 /// Item 181, from the user: "the thread dashboard doesn't update live with the
 /// modifications applied to the list pane. If I mark the thread as read, the
 /// dash still reports N unread".
diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index 28d3a8b..76ac0e8 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -109,6 +109,7 @@ private slots:
     void aConversationListsItsFirstMessageAsAChild();
     void aMessageRowDoesNotListItselfBeneathItself();
     void aMixedSelectionCarriesBothScopes();
+    void idLookupsFollowEveryChangeToTheRows();
 };
 
 static ThreadSummary makeThread(const QString &id, const QString &subject)
@@ -2355,5 +2356,53 @@ void TestThreadListModel::aMixedSelectionCarriesBothScopes()
     QCOMPARE(scope.threadIds, QStringList{ QStringLiteral("t2") });
 }
 
+void TestThreadListModel::idLookupsFollowEveryChangeToTheRows()
+{
+    // Item 211. Lookups by id go through a lazily rebuilt hash rather than a
+    // walk, so every way the rows can change has to invalidate it. Each step
+    // asks a question whose answer moved, so a stale index answers wrongly.
+    const QString a = QStringLiteral("a@example.org");
+    const QString c = QStringLiteral("c@example.org");
+    const QString d = QStringLiteral("d@example.org");
+    const QString e = QStringLiteral("e@example.org");
+    const QString t1 = QStringLiteral("t1");
+    const QString t2 = QStringLiteral("t2");
+
+    ThreadListModel model;
+    ThreadSummary one = makeThread(t1, QStringLiteral("one"));
+    one.firstMessageId = a;
+    ThreadSummary two = makeThread(t2, QStringLiteral("two"));
+    two.firstMessageId = c;
+    model.appendBatch({ one, two });
+    QCOMPARE(model.threadIdForMessage(a), t1);   // builds the index
+
+    // Expansion adds ids the index has never seen.
+    MessageNode first = makeNode(c, 0);
+    first.threadId = t2;
+    MessageNode reply = makeNode(d, 1);
+    reply.threadId = t2;
+    model.setThreadMessages(t2, { first, reply });
+    QCOMPARE(model.threadIdForMessage(d), t2);
+
+    // A removal renumbers: t2 moves from row 1 to row 0.
+    model.applyTagChange(t1, {}, { QStringLiteral("inbox") });
+    model.removeThreadsWithoutTag({ t1 }, QStringLiteral("inbox"));
+    QVERIFY(!model.hasThread(t1));
+    QVERIFY(model.threadIdForMessage(a).isEmpty());
+    QCOMPARE(model.threadCountFor(t2), 2);
+    model.applyMessageTagChange(d, { QStringLiteral("x") }, {});
+    QVERIFY(model.messageById(d).tags.contains(QStringLiteral("x")));
+
+    // reconcile() replaces a summary IN PLACE, with no structural signal.
+    two.firstMessageId = e;
+    two.tags = QStringList{ QStringLiteral("inbox") };
+    model.reconcile({ two });
+    QCOMPARE(model.threadIdForMessage(e), t2);
+
+    model.clear();
+    QVERIFY(!model.hasThread(t2));
+    QVERIFY(model.messageById(d).messageId.isEmpty());
+}
+
 QTEST_MAIN(TestThreadListModel)
 #include "test_threadlistmodel.moc"