Widgets themselves shouldn't have margins assigned.
<!-- interface-requires gtk+ 3.10 -->
<template class="GtkStatusbar" parent="GtkBox">
<property name="spacing">2</property>
- <property name="margin-start">10</property>
- <property name="margin-end">10</property>
- <property name="margin-top">6</property>
- <property name="margin-bottom">6</property>
<child>
<object class="GtkFrame" id="frame">
<property name="shadow-type">none</property>