It's a generic keyboard functionality that Zotero inherits from the Mozilla framework it uses -- I think it used to be more common than it is now, but you can still find it in a fair number of apps. There's are a whole bunch of standard keyboard functionality that isn't specifically documented, including more obvious ones such as tab & arrows to move between fields & items, and lesser known ones such as hitting a letter to advance to it in a sorted (middle panel) list.
It's a generic keyboard functionality that Zotero inherits from the Mozilla framework it uses
No, that one's us. Other functionality in the trees — such as the find-as-you-type that you mention, or right/left-arrow to expand/collapse parent rows — comes from Mozilla.
Some additional ones are listed here: https://www.zotero.org/support/tips_and_tricks