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, 10 insertions, 0 deletions

diff --git a/src/threadlistmodel.h b/src/threadlistmodel.h
index bc9fcbf..e39ade6 100644
--- a/src/threadlistmodel.h
+++ b/src/threadlistmodel.h
@@ -134,6 +134,16 @@ public:
 
     int rowCount(const QModelIndex &parent = {}) const override;
     int columnCount(const QModelIndex &parent = {}) const override;
+
+    /// Whether a thread row should offer an expander.
+    ///
+    /// Answered from totalCount rather than from the loaded children, and that
+    /// is what makes lazy loading possible at all: rowCount is 0 until the
+    /// worker has walked the thread, so a view left to infer this from rowCount
+    /// alone draws no expander, the user can never expand, and the replies are
+    /// never asked for. The count is already in the summary, so this costs
+    /// nothing.
+    bool hasChildren(const QModelIndex &parent = {}) const override;
     QVariant data(const QModelIndex &index, int role) const override;
     QVariant headerData(int section, Qt::Orientation orientation,
                         int role) const override;