While the icon appears on my desktop system (attachment 1), the icon does not appear on my laptop.
I copied the xfce4 configuration from my desktop to my laptop, but nothing changed.
Even if select the same display resolution on desktop and laptop nothing changes
xfce-panel 4.12: I just installed clipman 1.2.6 on my Gentoo system
and added it to the panel. The icon is blank. Clipman is there just
where it's supposed to be, and hovering over or clicking on that blank
icon does what is expected.
The attached .png shows a snapshot (sans pointer, for some reason) while
hovering over the blank clipman spot on the panel.
The plugin looks for an icon called "clipman" and falls back to "edit-paste". Neither icon is shipped by the plugin because it is assumed that "edit-paste" is present in a standard icon theme.
This doesn't always seem to be the case (partly because the Adwaita/Gnome icon theme was split into multiple packages by distros), so I guess we need to either change the icon names or ship fallback icons.