thin line remains after bar is hidden

[sukolyn]

hi,

using dzen2 0.9.5.14.488ab66-3 with herbstluftwm 0.9.6
when I togglehide or hide dzen2 configured to be at bottom, a thin line remains up where dzen2 was
here it's unhide state :

and once hidden :

is it dzen2 or herbstluftwm ?