This is an experiment with adding a filler column.
<child>
<object class="GtkColumnViewColumn" id="path">
<property name="title" translatable="yes">Path</property>
+ <property name="expand">1</property>
<property name="factory">
<object class="GtkSignalListItemFactory">
<signal name="setup" handler="setup_name_cb"/>
</property>
</object>
</child>
+ <child>
+ <object class="GtkColumnViewColumn" id="filler">
+ <property name="expand">1</property>
+ </object>
+ </child>
</object>
</child>
</object>