diff --git a/src/zenity.ui b/src/zenity.ui
index f6bc0c7..72be522 100644
--- a/src/zenity.ui
+++ b/src/zenity.ui
@@ -995,6 +995,7 @@
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
                 <property name="shadow_type">in</property>
+                <property name="expand">True</property>
                 <child>
                   <object class="GtkTreeView" id="zenity_tree_view">
                     <property name="visible">True</property>