Even though a Xfce panel is “hidden” (i.e. “Autohide” is enabled) 3 pixels of it is still visible. But there is an easy way to hide it completely (Requires Xfwm4’s compositor to be enabled):

  • Right click on the panel you want to hide, select “Customize panel”
  • Set “Transparency” to 100%
  • Click “Make active panel opaque”

The 3 pixels are now 100% transparent, but when the mouse cursor moves over the panel, the panel becomes opaque.