I came across a potential issue with GtkEventBox. Not sure if it is caused by a problem in the panel, indicator plugin, or gtk. Reporting it here for future reference. If used in a plugin running as a standalone executable, the event box respects panel's background transparency settings. If used in a plugin running as a module linked by a wrapper binary, the event box has a non-transparent background. I have "fixed" the issue by removing the event box from the plugin as in commit: http://git.xfce.org/panel-plugins/xfce4-indicator-plugin/commit/?h=andrzejr/tmp3&id=00633e1de8308a1456a46f8dc03f7baa2a62c30a
This is either invalid or fixed meanwhile.