iTunes is already an amalgamation of many different things. With iTunes 12.4, Apple has reinforced its piecemeal design further by reintroducing parts of iTunes 11 without properly considering all the edge-cases and window states. 12.4 adds a sidebar that replaces a popover UI control to manage views like Albums, Artists and Genres.
The sidebar is a resurrection from the days of iTunes 11. I like the sidebar better than the transient popover (OS X has enough screen real estate to allow such affordances) but it hasn’t been thought through. It’s shoddy and incomplete. There are distinct sidebar sources for Albums and Compilations and yet selecting Albums still shows Compilations in the detail view when you scroll down. Some media types do not hide the sidebar but have no sidebar items to choose between (Podcasts). Many media views have no sidebar at all, leading to jarring transitions between tabs, including all of Apple Music.
Again harkening back to iTunes 11, the Media Picker is now once again presented as a menu dropdown. In earlier versions of iTunes 12, the switcher for Music, Movies, TV Shows and such was presented as a mini tab bar, with a More button to reveal more. I actually prefer the new dropdown for overall usability as it features text labels alongside the glyphs. You can also edit the list to show just the items relevant to your library. However, it is now two clicks to change views which is a regression in efficiency. This is obviously frustrating if you context switch a lot but I don’t mind it — I rarely use iTunes for anything but music.
Most infuriatingly, iTunes has now made Compilations a second-class citizen in the library interface. There is now mandatory filtering, separating normal albums from compilations. To see this for yourself, click on Albums in the sidebar and scroll down. Previously, all of the albums and compilations would appear in one grid. With iTunes 12.4, they are now sectioned independently. This is so frustrating as much of my library consists of compilations and no longer participate in the normal album ordering.
As far as I can tell, there is no way to revert to the previous layout whilst maintaining a sort order by artist name. If you want to coalesce them and don’t mind sacrificing Artist ordering, change the View Options to sort by title.
In summary, iTunes continues to suck. It’s held back by its ageing codebase and the necessity for it to be a cross-platform program. A good version of the desktop app probably isn’t going to happen until Apple splits all the cogent functions into separate apps, like on iOS with dedicated apps for Music, Movies and more. I look forward to the Music.app revamp in iOS 10 to see an unconstrained representation of Apple’s vision for music software.