it looks like a bug james, xul and luna.. i tested with classic mode and it worked fine.. with spacing maintained no matter how many i added.
i hope it's alright, i took liberty in uploading your snapshot so others can view it easily:
without luna theme:
i can test with luna if i could but.. it's disabled on so many levels, it will be painful to enable it.. i think i also hexed-out some of its bitmaps inside shell libraries years ago.(dumb i know).. can you please try testing without luna or if someone has luna can test and see
(i don't even have themes in display)