Created attachment 8837 Screenshow, how it looks like It was missed some time ago, and now, panel looks not very nice.
Sorry but even from your screenshot I'm not sure I understand what you mean.
I mean the little vertical top and bottom padding for elements of panel. First of all it is a windows buttons . They are uses all available vertical space of panel. Workplace switcher has similar look. I think, all elements on panel should have a little padding (or margin if you like). On screenshot you can see 1 pixel lines around a terminal window button painted by me.
This is up to the Gtk theme, so nothing the panel would handle or enforce. You can either decide to modify your Gtk theme or put a gtk.css file in your $HOME/.config/gtk-3.0/ directory adding a margin. .xfce4-panel .tasklist button { margin: 1px; }