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, 29 insertions, 0 deletions

diff --git a/src/threadlistmodel.cpp b/src/threadlistmodel.cpp
index 0658b90..df083fc 100644
--- a/src/threadlistmodel.cpp
+++ b/src/threadlistmodel.cpp
@@ -151,6 +151,35 @@ int ThreadListModel::rowCount(const QModelIndex &parent) const
     return m_threads.at(parent.row()).children.size();
 }
 
+bool ThreadListModel::hasChildren(const QModelIndex &parent) const
+{
+    if (!parent.isValid())
+        return !m_threads.isEmpty();
+
+    // A message row is always a leaf. Reply depth is drawn from the node's own
+    // depth, not from further nesting, so nothing hangs under a reply.
+    if (parent.parent().isValid())
+        return false;
+
+    if (parent.column() != 0)
+        return false;
+
+    if (parent.row() < 0 || parent.row() >= m_threads.size())
+        return false;
+
+    const ThreadNode &node = m_threads.at(parent.row());
+
+    // Once loaded the children are the truth, including "there are none", which
+    // is how a thread whose totalCount counted duplicates stops offering an
+    // expander that opens onto nothing.
+    if (node.loaded)
+        return !node.children.isEmpty();
+
+    // Before loading, the summary's count is all there is. A thread of one
+    // message has no replies and must not offer an expander.
+    return node.summary.totalCount > 1;
+}
+
 int ThreadListModel::columnCount(const QModelIndex &parent) const
 {
     // Every level has the same columns. Returning 0 for a valid parent, as the