Keys/App/UI/MainWindow.xml

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <object class="AdwWindow" id="main_window">
  <property name="width-request">280</property>
  <property name="height-request">200</property>
  <property name="default-width">800</property>
  <property name="default-height">500</property>
  <child>
    <object class="AdwBreakpoint">
      <condition>max-width: 500sp</condition>
      <setter object="split_view" property="collapsed">True</setter>
      <setter object="show_sidebar_button" property="visible">True</setter>
    </object>
  </child>
  <property name="content">
    <object class="AdwOverlaySplitView" id="split_view">
      <property name="min-sidebar-width">200</property>
      <property name="max-sidebar-width">200</property>
      <property name="show-sidebar"  
            bind-source="show_sidebar_button"  
            bind-property="active"  
            bind-flags="sync-create|bidirectional"/>
      <property name="sidebar">
        <object class="AdwNavigationPage">
          <property name="title" translatable="yes">Sidebar</property>
          <property name="child">
            <object class="AdwToolbarView">
              <child type="top">
                <object class="AdwHeaderBar" />
              </child>
              <property name="content">
                <object class="GtkLabel">
                  <property name="label">I am a sidebar</property>
                </object>
              </property>
            </object>
          </property>
        </object>
      </property>
      <property name="content">
        <object class="AdwNavigationPage">
          <property name="title" translatable="yes">Content</property>
          <property name="child">
            <object class="AdwToolbarView">
              <child type="top">
                <object class="AdwHeaderBar">
                  <child type="start">  
                    <object class="GtkToggleButton" id="show_sidebar_button">  
                      <property name="icon-name">sidebar-show-symbolic</property>  
                      <property name="active">True</property>  
                      <property name="visible">False</property>
                    </object>  
                  </child>
                </object>
              </child>
              <property name="content">
                <object class="GtkLabel">
                  <property name="label">Hello world</property>
                </object>
              </property>
            </object>
          </property>
        </object>
      </property>
    </object>
  </property>
</object>
</interface>