Add missing hasChildren() implementation.

Summary:
QAbstractProxyModel forwards hasChildren() to the source model, which
will return a wrong value for our tree-restructuring model. Current
views/proxies don't call hasChildren(), which is how this oversight
happened, but implementing it improves model hygiene - could blow up
in the future otherwise.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1799

libtaskmanager/taskgroupingproxymodel.cpp

@@ -599,6 +599,15 @@ int TaskGroupingProxyModel::rowCount(const QModelIndex &parent) const
     return d->rowMap.count();
 }
 
+bool TaskGroupingProxyModel::hasChildren(const QModelIndex &parent) const
+{
+    if ((parent.model() && parent.model() != this) || !sourceModel()) {
+        return false;
+    }
+
+    return rowCount(parent);
+}
+
 int TaskGroupingProxyModel::columnCount(const QModelIndex &parent) const
 {
     Q_UNUSED(parent)

libtaskmanager/taskgroupingproxymodel.h

@@ -62,6 +62,7 @@ public:
     QModelIndex mapToSource(const QModelIndex &proxyIndex) const override;
 
     int rowCount(const QModelIndex &parent = QModelIndex()) const override;
+    bool hasChildren(const QModelIndex &parent = QModelIndex()) const override;
     int columnCount(const QModelIndex &parent = QModelIndex()) const override;
 
     QVariant data(const QModelIndex &proxyIndex, int role) const override;