xfce4-generic slider does not rotate its label in the vertical mode and does not resize correctly when the panel orientation/mode changes. Note that xfce4-panel v.4.9.0+ has 2 vertical modes: a deskbar mode with labels oriented horizontally, and a vertical mode where all labels are supposed to be oriented vertically. More details: http://wiki.xfce.org/releng/4.10/roadmap/xfce4-panel https://wiki.xfce.org/dev/panel-deskbar-mode
Created attachment 4234 Label orientation, resizing after panel orientation change. I tried to keep the changes minimum. Perhaps orientation_changed (mode_changed) callbacks could be refactored in order to avoid duplication of code. Also (just a comment), it is not necessary to remove a Gtk(V/H)Box and add it again to change its orientation. It can be done with: gtk_orientable_set_orientation(GTK_ORIENTABLE(box), xfce_panel_plugin_get_orientation(plugin));
I made the code shared between the two callbacks and used your suggestion about changing the orientation. Let me know if I messed something up.
Work well here (with xfce4-panel 4.9). Thanks.