GtkBox has unwanted spacing
GNOME To Do has a double headerbar, inside a GtkBox. Recently, it started showing black borders around it. This is reproducible with the following GtkBuilder:
<interface>
<template class="BadheaderbarWindow" parent="GtkApplicationWindow">
<child type="titlebar">
<object class="GtkBox">
<child>
<object class="GtkHeaderBar" id="headerBar">
<property name="hexpand">true</property>
</object>
</child>
<child>
<object class="GtkSeparator">
<property name="orientation">vertical</property>
</object>
</child>
<child>
<object class="GtkHeaderBar" id="headerBar2">
<property name="hexpand">true</property>
<property name="show-title-buttons">true</property>
</object>
</child>
</object>
</child>
<child>
<object class="GtkLabel" id="label">
<property name="label">Hello, World!</property>
<attributes>
<attribute name="weight" value="bold"/>
<attribute name="scale" value="2"/>
</attributes>
</object>
</child>
</template>
</interface>
This is how it looks:
This is probably related to GtkBoxLayout.