feat(ui): load a thread's replies when its row is expanded

Replies are fetched on expansion rather than with the query: walking the reply
tree of every thread in a 10k-thread result would cost more than the query and
almost none of it would be looked at.

hasChildren is what makes that lazy loading work, and its absence would have
shipped the feature unreachable. rowCount is 0 until the worker has walked the
thread, so a view left to infer the expander from rowCount alone draws none, the
user can never expand, and the replies are never requested. It answers from the
summary's totalCount before loading and from the children afterwards, so a
thread whose count included duplicates stops offering an expander that opens
onto nothing.

onThreadTreeLoaded reads the thread id from the reply rather than remembering it
from the request. Two expansions can be in flight at once, and pairing them by
order would attach one thread's replies to the other.

1 files changed, 41 insertions, 0 deletions

diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index 74b8dd1..3d131cf 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -32,6 +32,7 @@ private slots:
     void repliesBecomeChildRowsUnderTheirThread();
     void messageRowsShowTheirOwnSenderAndSubject();
     void reloadingAThreadReplacesItsRepliesRatherThanRepeatingThem();
+    void anUnexpandedMultiMessageThreadOffersAnExpander();
     void scopeFollowsTheSelectedRowKind();
     void scopeCountsEveryMessageOfAnUnexpandedThread();
     void scopeHonoursAMixedSelectionWithoutEscalating();
@@ -196,6 +197,46 @@ void TestThreadListModel::reloadingAThreadReplacesItsRepliesRatherThanRepeatingT
     Q_UNUSED(tester);
 }
 
+void TestThreadListModel::anUnexpandedMultiMessageThreadOffersAnExpander()
+{
+    // This is what makes lazy loading work at all. rowCount is 0 until the
+    // worker has walked the thread, so a view inferring the expander from
+    // rowCount alone draws none, the user can never expand, and the replies are
+    // never requested. hasChildren answers from the summary's count instead.
+    ThreadListModel model;
+    ThreadSummary many = makeThread(QStringLiteral("t1"),
+                                    QStringLiteral("Has replies"));
+    many.totalCount = 4;
+    ThreadSummary lone = makeThread(QStringLiteral("t2"),
+                                    QStringLiteral("Single message"));
+    lone.totalCount = 1;
+    model.appendBatch({ many, lone });
+
+    const QModelIndex withReplies = model.index(0, 0, QModelIndex());
+    const QModelIndex single = model.index(1, 0, QModelIndex());
+
+    // Guard: neither is expanded, so this really is the unloaded case.
+    QCOMPARE(model.rowCount(withReplies), 0);
+    QCOMPARE(model.rowCount(single), 0);
+
+    QVERIFY(model.hasChildren(withReplies));
+    QVERIFY(!model.hasChildren(single));
+
+    // Once loaded the children are the truth, including "there are none": a
+    // thread whose count included duplicates must stop offering an expander
+    // that opens onto nothing.
+    model.setThreadMessages(QStringLiteral("t1"),
+                            { makeNode(QStringLiteral("m0@example.org"), 0) });
+    QVERIFY(!model.hasChildren(withReplies));
+
+    // A message row is always a leaf.
+    model.setThreadMessages(QStringLiteral("t1"),
+                            { makeNode(QStringLiteral("m0@example.org"), 0),
+                              makeNode(QStringLiteral("m1@example.org"), 1) });
+    QVERIFY(model.hasChildren(withReplies));
+    QVERIFY(!model.hasChildren(model.index(0, 0, withReplies)));
+}
+
 void TestThreadListModel::scopeFollowsTheSelectedRowKind()
 {
     ThreadListModel model;