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.
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:
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.