refactor(model): convert ThreadListModel to QAbstractItemModel

A table cannot indent or expand, so message rows need a tree. This task changes
only the base class and the index plumbing: no children are produced yet, so the
30 pre-existing tests in test_threadlistmodel are the regression net proving a
thread row still behaves exactly as it did, and QAbstractItemModelTester checks
the index/parent round trip a hand-written assertion would miss.

Two things the table version could leave wrong and a tree cannot. columnCount
returned 0 for a valid parent, which would give message rows no columns and
render them blank. And rowCount now answers only for column 0, since a tree
takes one set of children per row and offering them under every column draws an
expander in each.

The model stays two levels deep even though replies carry a reply depth of their
own. The visual nesting past the first level comes from that depth, not from
further parent-child structure, so no index calculation has to recurse.

1 files changed, 34 insertions, 0 deletions

diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index e2ca09f..9684637 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -28,6 +28,7 @@ class TestThreadListModel : public QObject
     Q_OBJECT
 private slots:
     void messageNodeHoldsDisplayFacts();
+    void rootRowsSurviveTheTreeConversion();
     void startsEmpty();
     void accountKeysComeFromTheAccountTags();
     void accountKeysCoverAThreadSpanningTwoAccounts();
@@ -142,6 +143,39 @@ void TestThreadListModel::messageNodeHoldsDisplayFacts()
     QVERIFY(!fresh.isUnread());
 }
 
+void TestThreadListModel::rootRowsSurviveTheTreeConversion()
+{
+    // The point of this test is NOT the tree. It is that converting the base
+    // class from QAbstractTableModel changed nothing a thread row does: a table
+    // answers index() and parent() too, just trivially, and every existing test
+    // in this file is the real regression net beside it.
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("A subject")) });
+
+    // A tree model reports its roots under an INVALID parent.
+    QCOMPARE(model.rowCount(QModelIndex()), 1);
+    QCOMPARE(model.columnCount(QModelIndex()), ThreadListModel::ColumnCount);
+
+    const QModelIndex root =
+        model.index(0, ThreadListModel::SubjectColumn, QModelIndex());
+    QVERIFY(root.isValid());
+    QVERIFY(!model.parent(root).isValid());
+    QCOMPARE(model.data(root, ThreadListModel::ThreadIdRole).toString(),
+             QStringLiteral("t1"));
+
+    // No children until a thread's messages are asked for. An expander drawn
+    // over a thread whose replies were never loaded would open onto nothing.
+    QCOMPARE(model.rowCount(root), 0);
+
+    // Qt's own conformance check. It walks index/parent/rowCount for
+    // consistency and catches the classic tree-model faults, such as a parent()
+    // that does not round-trip, which a hand-written assertion misses.
+    QAbstractItemModelTester tester(
+        &model, QAbstractItemModelTester::FailureReportingMode::Warning);
+    Q_UNUSED(tester);
+}
+
 void TestThreadListModel::startsEmpty()
 {
     ThreadListModel model;