configure max width in tab label to avoid too big tabs

data/gui/message_window.ui

@@ -44,6 +44,7 @@
             <property name="xalign">0</property>
             <property name="use_markup">True</property>
             <property name="ellipsize">end</property>
+            <property name="max_width_chars">9</property>
           </object>
           <packing>
             <property name="position">1</property>