Created attachment 4207 Makes the event box window invisible. The event box is visible, so the background may not match the panel in certain themes. Use an invisible event box instead.
Created attachment 4216 Screenshot I can't reproduce. Please see attached screenshot. The "113.0" is Genmon.
Created attachment 4218 Example of the problem I'm not sure what the conditions are, but it's been shown in other bugs that the visible eventbox background is at fault (see bugs 7056 and 8449), and making that change fixes it here.
I've committed your patch. Thank you. http://git.xfce.org/panel-plugins/xfce4-genmon-plugin/commit/?id=2a488de06506ff297ae6e686f5f2640e38ba9fdd
For future reference, you can use git commit --author="Author's Name <email@address.com>" to set the author of the patch in the commit.