nettop has VGA1 and LVDS1, monitor plug in VGA1

cat ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml
 
<?xml version="1.0" encoding="UTF-8"?>

<channel name="xfce4-desktop" version="1.0">
  <property name="desktop-icons" type="empty">
    <property name="file-icons" type="empty">
      <property name="show-trash" type="bool" value="false"/>
      <property name="show-removable" type="bool" value="false"/>
      <property name="show-filesystem" type="bool" value="false"/>
      <property name="show-home" type="bool" value="true"/>
    </property>
    <property name="icon-size" type="uint" value="42"/>
  </property>
  <property name="backdrop" type="empty">
    <property name="screen0" type="empty">
      <property name="monitor0" type="empty">
        <property name="image-path" type="string" value="/usr/share/wallpapers/yz03b.jpg"/>
        <property name="last-image" type="string" value="/usr/share/wallpapers/yz03b.jpg"/>
        <property name="last-single-image" type="string" value="/usr/share/wallpapers/yz03b.jpg"/>
        <property name="image-style" type="int" value="3"/>
      </property>
      <property name="monitor1" type="empty">
        <property name="image-path" type="string" value="/usr/share/backgrounds/xfce/xfce-blue.jpg"/>
        <property name="last-image" type="string" value="/usr/share/backgrounds/xfce/xfce-blue.jpg"/>
        <property name="last-single-image" type="string" value="/usr/share/backgrounds/xfce/xfce-blue.jpg"/>
      </property>
    </property>
  </property>
</channel>

after boot I see xfce-blue.jpg drawed over yz03b.jpg. if i set one wallpaper of them by hand - all drawed correctly.
<property name="xinerama-stretch" type="bool" value="true"/>
fix problem, but it bad workaround.