Whenever tabs are shown in a window, the tabs have a darker edge on the right side and the inactive tabs are separated from the content by a light borderline. If you look closely at the right side of the active tab, you will see a single light pixel near the bottom, similar to the light borderline. It's one of those things which are hard to notice, but once you've seen it, you can't help but notice it.
I'm not sure if it is intended or not but it looks out of place, being the only light pixel in the otherwise dark border. I suppose it is supposed to give some perspective to make the tab look like it's standing out, but I just thinks it looks misplaced. It would probably have worked better had it been moved to the right side, where it would be a part of the light borderline.
Oh, and yes I know this is probably the tiniest minor annoyance I have reported. I have no problem with this being placed pretty near the end on the ToDo-list. :)
I have been trying to find that in the graphics files (widelands/pics) but it doesn't seem to be there.
So, I guess the facette is made in code.
OT: What is a "Treeherder" :) can't help thinking of Ents.... boarumhumrum....