feat(model): expose a thread's replies as child rows

setThreadMessages drops the depth-0 message: it is the thread's first message
and the root row already stands for it. Keeping it would show a thread of seven
as one root and seven children, contradicting the reply count the row
advertises. Calling again replaces rather than appends, so a thread reloaded
after a sync does not list its replies twice.

A message row reports its own sender and subject, not the thread's. That is the
mistake worth guarding: the thread's author summary usually contains the first
sender too, so reading it renders something plausible for the root's own reply
and wrong for every other one. Mutation-checked, and the wrong version returns
'Alice' where 'Bob' belongs.

Child rows carry no tag pills. The strip is a row-wide band of the thread's
tags; one under each reply would stripe the list and repeat identical tags down
the expansion.

3 files changed, 268 insertions, 5 deletions

diff --git a/src/threadlistmodel.cpp b/src/threadlistmodel.cpp
index dabc447..3bc02d6 100644
--- a/src/threadlistmodel.cpp
+++ b/src/threadlistmodel.cpp
@@ -169,12 +169,60 @@ QVariant ThreadListModel::data(const QModelIndex &index, int role) const
         return {};
     }
 
-    // Message rows are handled in Task 4; until then only thread rows exist and
-    // a child index cannot be produced. The bound is checked against the thread
-    // list only after establishing this IS a thread row, since a child row's
+    // A message row. Handled before the bounds check below, since a child row's
     // number indexes its siblings, not m_threads.
-    if (index.parent().isValid())
-        return {};
+    if (isMessageRow(index)) {
+        const MessageNode node = messageAt(index);
+        if (node.messageId.isEmpty())
+            return {};
+
+        switch (role) {
+        case IsMessageRole:
+            return true;
+        case MessageIdRole:
+            return node.messageId;
+        case MessageDepthRole:
+            return node.depth;
+        case ThreadIdRole:
+            // A message row still belongs to a thread, and a caller that only
+            // needs the containing thread must not have to walk up itself.
+            return node.threadId;
+        case TagsRole:
+        case PillTagsRole:
+            // No strip under a child row: the strip is a ROW-wide band carrying
+            // the thread's tags, and one under every reply would stripe the
+            // list and repeat the same tags down the whole expansion.
+            return QStringList();
+        case PillColoursRole:
+            return QVariantList();
+        case AccountLabelRole:
+            return QString();
+        case Qt::DisplayRole:
+            switch (index.column()) {
+            case AuthorsColumn:
+                // The REPLY's sender, not the thread's author summary. Reading
+                // the thread's fields here would look almost right, since the
+                // first sender usually appears in both.
+                return node.from;
+            case SubjectColumn:
+                return node.subject;
+            case DateColumn:
+                return node.date.toString(QStringLiteral("yyyy-MM-dd hh:mm"));
+            case AttachmentColumn:
+                return node.hasAttachment() ? attachmentGlyph() : QString();
+            case FlagColumn:
+                return node.isFlagged() ? flagGlyph() : QString();
+            default:
+                return {};
+            }
+        case Qt::ForegroundRole:
+            // Same rule as a thread row: read recedes, unread stays at the
+            // palette's own colour.
+            return node.isUnread() ? QVariant() : QVariant(readColour());
+        default:
+            return {};
+        }
+    }
 
     if (index.row() >= m_threads.size())
         return {};
@@ -184,6 +232,19 @@ QVariant ThreadListModel::data(const QModelIndex &index, int role) const
     if (role == ThreadIdRole)
         return thread.threadId;
 
+    // Answered rather than left to fall through as an invalid QVariant. An
+    // invalid one converts to false and an empty string anyway, so the
+    // behaviour is the same, but a role the model never mentions is a latent
+    // bug the next reader has to prove is harmless.
+    if (role == IsMessageRole)
+        return false;
+
+    if (role == MessageIdRole)
+        return QString();
+
+    if (role == MessageDepthRole)
+        return 0;
+
     if (role == TagsRole)
         return thread.tags;
 
@@ -376,6 +437,64 @@ void ThreadListModel::clear()
     endResetModel();
 }
 
+void ThreadListModel::setThreadMessages(const QString &threadId,
+                                        const QVector<MessageNode> &nodes)
+{
+    for (int row = 0; row < m_threads.size(); ++row) {
+        if (m_threads.at(row).summary.threadId != threadId)
+            continue;
+
+        const QModelIndex parent = index(row, 0, QModelIndex());
+
+        // Replace, not append. A thread reloaded after a sync would otherwise
+        // list every reply twice.
+        if (!m_threads.at(row).children.isEmpty()) {
+            beginRemoveRows(parent, 0, m_threads.at(row).children.size() - 1);
+            m_threads[row].children.clear();
+            endRemoveRows();
+        }
+
+        QVector<MessageNode> children;
+        children.reserve(nodes.size());
+        for (const MessageNode &node : nodes) {
+            if (node.depth > 0)
+                children.append(node);
+        }
+
+        if (!children.isEmpty()) {
+            beginInsertRows(parent, 0, children.size() - 1);
+            m_threads[row].children = children;
+            endInsertRows();
+        }
+
+        // Set even when there are no replies: that is the difference between a
+        // single-message thread and one whose replies were never fetched.
+        m_threads[row].loaded = true;
+        return;
+    }
+}
+
+bool ThreadListModel::isMessageRow(const QModelIndex &index) const
+{
+    return index.isValid() && index.parent().isValid();
+}
+
+MessageNode ThreadListModel::messageAt(const QModelIndex &index) const
+{
+    if (!isMessageRow(index))
+        return {};
+
+    const int threadRow = index.parent().row();
+    if (threadRow < 0 || threadRow >= m_threads.size())
+        return {};
+
+    const QVector<MessageNode> &children = m_threads.at(threadRow).children;
+    if (index.row() < 0 || index.row() >= children.size())
+        return {};
+
+    return children.at(index.row());
+}
+
 ThreadSummary ThreadListModel::threadAt(int row) const
 {
     if (row < 0 || row >= m_threads.size())
diff --git a/src/threadlistmodel.h b/src/threadlistmodel.h
index 1aa0271..a7ce5d3 100644
--- a/src/threadlistmodel.h
+++ b/src/threadlistmodel.h
@@ -84,6 +84,18 @@ public:
         /// model because it owns the TagColors instance; a delegate reading
         /// config itself would be a second source of truth.
         PillColoursRole,
+
+        /// True when the row is a MESSAGE row rather than a thread root.
+        /// Drives both the action scope and whether the view paints a tag
+        /// strip under the row.
+        IsMessageRole,
+
+        /// The message id behind a message row. Empty on a thread root.
+        MessageIdRole,
+
+        /// The message's reply depth, for the view's indentation. 1 for a
+        /// direct reply, since depth 0 is the root row itself.
+        MessageDepthRole,
     };
 
     /// Row fill for a thread tagged `deleted`, and for one tagged `spam`.
@@ -131,6 +143,23 @@ public:
 
     ThreadSummary threadAt(int row) const;
 
+    /// Fills in a thread's message rows once the worker has walked its tree.
+    ///
+    /// The depth-0 message is dropped: it is the thread's first message and the
+    /// ROOT row already stands for it. Keeping it would show a thread of seven
+    /// as one root and seven children, contradicting the reply count the row
+    /// advertises. Calling again replaces the rows rather than appending, so a
+    /// thread reloaded after a sync does not list its replies twice.
+    void setThreadMessages(const QString &threadId,
+                           const QVector<MessageNode> &nodes);
+
+    /// True when the index is a message row rather than a thread root.
+    bool isMessageRow(const QModelIndex &index) const;
+
+    /// The message row's node, or a default-constructed one for any index that
+    /// is not a message row.
+    MessageNode messageAt(const QModelIndex &index) const;
+
     /// The account keys behind a thread's account tags, for item 49's
     /// per-account sync.
     ///
diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index 9684637..b4be527 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -29,6 +29,9 @@ class TestThreadListModel : public QObject
 private slots:
     void messageNodeHoldsDisplayFacts();
     void rootRowsSurviveTheTreeConversion();
+    void repliesBecomeChildRowsUnderTheirThread();
+    void messageRowsShowTheirOwnSenderAndSubject();
+    void reloadingAThreadReplacesItsRepliesRatherThanRepeatingThem();
     void startsEmpty();
     void accountKeysComeFromTheAccountTags();
     void accountKeysCoverAThreadSpanningTwoAccounts();
@@ -78,6 +81,118 @@ static ThreadSummary makeThread(const QString &id, const QString &subject)
     return t;
 }
 
+static MessageNode makeNode(const QString &id, int depth,
+                            const QString &from = QStringLiteral("Alice"),
+                            const QString &subject = QStringLiteral("Re: Hi"))
+{
+    MessageNode n;
+    n.messageId = id;
+    n.threadId = QStringLiteral("t1");
+    n.from = from;
+    n.subject = subject;
+    n.date = QDateTime::fromSecsSinceEpoch(1750000000);
+    n.depth = depth;
+    return n;
+}
+
+void TestThreadListModel::repliesBecomeChildRowsUnderTheirThread()
+{
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("A subject")) });
+
+    // Depth 0 is the thread's FIRST message and belongs on the root row, not in
+    // the children: the user's model is "N replies", so a thread of three shows
+    // one root and two children.
+    model.setThreadMessages(QStringLiteral("t1"),
+                            { makeNode(QStringLiteral("m0@example.org"), 0),
+                              makeNode(QStringLiteral("m1@example.org"), 1),
+                              makeNode(QStringLiteral("m2@example.org"), 2) });
+
+    const QModelIndex root = model.index(0, 0, QModelIndex());
+    QCOMPARE(model.rowCount(root), 2);
+
+    const QModelIndex child =
+        model.index(0, ThreadListModel::SubjectColumn, root);
+    QVERIFY(child.isValid());
+    QCOMPARE(model.parent(child), model.index(0, 0, QModelIndex()));
+
+    QVERIFY(model.data(child, ThreadListModel::IsMessageRole).toBool());
+    QCOMPARE(model.data(child, ThreadListModel::MessageIdRole).toString(),
+             QStringLiteral("m1@example.org"));
+
+    // A message row still belongs to a thread, so a caller that only needs the
+    // containing thread does not have to walk up itself.
+    QCOMPARE(model.data(child, ThreadListModel::ThreadIdRole).toString(),
+             QStringLiteral("t1"));
+
+    // A thread root is not a message row and carries no message id.
+    QVERIFY(!model.data(root, ThreadListModel::IsMessageRole).toBool());
+    QVERIFY(model.data(root, ThreadListModel::MessageIdRole)
+                .toString().isEmpty());
+
+    QAbstractItemModelTester tester(
+        &model, QAbstractItemModelTester::FailureReportingMode::Warning);
+    Q_UNUSED(tester);
+}
+
+void TestThreadListModel::messageRowsShowTheirOwnSenderAndSubject()
+{
+    // A reply's row shows the REPLY's sender, not the thread's author summary.
+    // Reading the thread's fields for a child row is the obvious mistake and
+    // would look almost right, since the first sender is usually in both.
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("A subject")) });
+    model.setThreadMessages(
+        QStringLiteral("t1"),
+        { makeNode(QStringLiteral("m0@example.org"), 0),
+          makeNode(QStringLiteral("m1@example.org"), 1,
+                   QStringLiteral("Bob <bob@example.org>"),
+                   QStringLiteral("Re: A subject")) });
+
+    const QModelIndex root = model.index(0, 0, QModelIndex());
+    const QModelIndex authors =
+        model.index(0, ThreadListModel::AuthorsColumn, root);
+    const QModelIndex subject =
+        model.index(0, ThreadListModel::SubjectColumn, root);
+
+    QCOMPARE(model.data(authors, Qt::DisplayRole).toString(),
+             QStringLiteral("Bob <bob@example.org>"));
+    QCOMPARE(model.data(subject, Qt::DisplayRole).toString(),
+             QStringLiteral("Re: A subject"));
+
+    // No tag strip under a child row. The strip is a row-wide band carrying the
+    // THREAD's tags; one under every reply would stripe the list and repeat the
+    // same tags down the whole expansion.
+    QVERIFY(model.data(subject, ThreadListModel::PillTagsRole)
+                .toStringList().isEmpty());
+}
+
+void TestThreadListModel::reloadingAThreadReplacesItsRepliesRatherThanRepeatingThem()
+{
+    // A thread reloaded after a sync must not end up listing its replies twice.
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("A subject")) });
+
+    const QVector<MessageNode> nodes{
+        makeNode(QStringLiteral("m0@example.org"), 0),
+        makeNode(QStringLiteral("m1@example.org"), 1)
+    };
+
+    model.setThreadMessages(QStringLiteral("t1"), nodes);
+    const QModelIndex root = model.index(0, 0, QModelIndex());
+    QCOMPARE(model.rowCount(root), 1);
+
+    model.setThreadMessages(QStringLiteral("t1"), nodes);
+    QCOMPARE(model.rowCount(root), 1);
+
+    QAbstractItemModelTester tester(
+        &model, QAbstractItemModelTester::FailureReportingMode::Warning);
+    Q_UNUSED(tester);
+}
+
 void TestThreadListModel::accountKeysComeFromTheAccountTags()
 {
     // Item 49 reads this to decide which mbsync channels a sync needs. Only