The smartbookmark entry widget does not recieve focus on pressing the left mouse button so it is impossible to use it. This happens with the most recent git version of xfce4-smartbookmark-plugin and xfce4-panel 4.8.1.
Created attachment 3465 fix focusing the entry widget Apparently the problem is in the custom Xlib calls, replacing it with the xfce_panel_plugin_focus_widget() xfce4-panel API call fixes the issue and seems to be the right thing anyway.
The patch works great. Applied in 0.4.4, thanks a lot.