I'm not sure if this is just a QtWidgets thing more than a Picard thing... but in the UI of OS X applications (Finder, for example), when a directory (or collection?) of items is highlighted, pressing the "Right Arrow" key will expand the UI element into a list of sub-items (files and subdirectories in the case of Finder, a Tracklist of Recordings in the case of Picard.) While, with the same top-level item highlighted, a "Left Arrow" keystroke will collapse the list.

In Picard, this only works on the top level item (The "Album" Release). A press of Left or Right arrow while a recording is highlighted will, maybe, only scroll the panel left or right.

In OS X, pressing "Left Arrow" while a sub item is highlighted, will move the selection highlight up to the top level item (The parent directory in the case of Finder, for example) Then a second press of "Left Arrow" will collapse the item.

In Picard, I'm constantly frustrated whenever I've been "Down Arrow" scrolling though the list of attached recordings, so I can read the "Original Value" vs. "New Value" of each track, and I want to quickly go back up to the top of the tracklist, and/or collapse this album, so I have more room to view the other albums... and I hit "Left Arrow"... and nothing happens. So I take my hand off the keyboard, and scroll up to the top, and then carefully position the pointer on the tiny triangle, and then carefully click without wiggling the trackpad at all. 

This would be so much faster and easier if I could just hit "Left Arrow" twice.