Since the commit: "commit 8fc415c498401f94d800564275f9d5db7a674c4a Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at> Date: Sun Mar 22 13:39:19 2015 +0100 Simplify icon loading for the panel plugin" The plugin button size is incorrect (too wide in horizontal mode, too tall in vertical/deskbar mode) if the panel is set to a multiple row layout.
Hm, I'm not entirely sure what's causing this – and after all, you're the expert for panel-rows ;) - but maybe it's the fact that I enforced the icon size with gtk_image_set_pixel_size. Maybe enforcing a square size for the plugin's button would help..?
A quick test with gtkinspector showed that it could be fixed by setting width/height requests for the plugin button, however, I'm not yet convinced that is the best way to solve this.
Fix pushed: http://git.xfce.org/xfce/xfce4-power-manager/commit/?id=fad62e2ae968eb1dde59fd43d615d4547872ea63
Closing this report then.