ZO_ScrollList issue - scroll bar not appearing

[merlight]

So I have a ZO_ScrollList and put ZO_SelectableLabel rows in it.

list:GetHeight() == 677
list.contents:GetHeight() == 677
list.controlHeight == 28
each label's height is 28

677 / 28 == 24.178571428571427 ... which means 24 labels fit without scrolling.

If there are 25 labels in the list, scroll bar doesn't appear, and the last label is partially visible (that's 0.17857 of its height).

If there are 26 labels, scroll bar does appear and work, but...
Thumb texture doesn't fit between Up/Down buttons. Button height is 16, so maximum allowable thumb height should be 677-2*16 = 645 (which happens to be exactly list.scrollbar:GetHeight()), but its actual height is 666.96838378906 and thus runs beyond the buttons!

If there are 27 labels, everything looks okay.

Did I do something stupid, or is there a bug in ZO_ScrollList?

edit: doing it on PTS