After a long (loooooong) time, Tracker has finally been switched to use the Layout Kit. Waddlesplash posted tonight over at the Haiku site that he has now merged his work on this branch, ending (well, it's still WIP) what started back in 2012.
This change is mostly in the background and may be unnoticed by the users (one trick is to spot the new icons in the navigation bar), but it's quite important moving forward, simplifying code and paving the way for future changes.
More details are available at the blog post above and after you update Haiku to the latest hrev and give it a spin, don't forget to file bug reports if you come across them. Nice work Waddleslapsh.