I tried unsuccessfully to remove the border on a Treeview. I first tried copying the default layout and changing "-border 1" to "-border 0" but that appears to have no effect (only tested this on the mac).
If I omit the Treeview.field element entirely, it seems that the Treeview.padding and/or the Treeview.textarea elements aren't properly expanding to fill the cavity. When I remove the field element I've left with one background color surrounding the listbox elements and another that fills up the empty space after the last element.
In the current implementation, the "field" element is responsible for both drawing the border and filling the background area with the window color (typically white). If you omit it, the border goes away (as desired) but the background will be filled with the frame color (typically gray) (not desired).
Either: [ttk::style configure Treeview -borderwidth 0] (not effective in all themes), or remove the field element from the layout and use [ttk::style configure Treeview -background $windowColor]. One or the other should work.