Yosemite switched the UI font to Helvetica Neue, but it's not quite the same font that you get when you select Helvetica Neue from FontBook or in any text field. There's a bit more space between characters in the UI font.
FontBook Helvetica Neue Light sample above a window title bar with the same content at the same size
I've looked around enough to find
/System/Library/Fonts/HelveticaNeueDeskInterface.ttc, which is distinct from
/System/Library/Fonts/HelveticaNeue.dfont, the actual HelveticaNeue that I can use. However, the DeskInterface incarnation doesn't appear to be usable by any means I can find.
I ask because there's been a growing number of applications built with web UIs (e.g. Atom, Visual Studio Code), and some (VS Code) try to match the OS by using "HelveticaNeue" as the UI font. Unfortunately, the HelveticaNeue text font is exceptionally poor on low DPI screens without the extra spacing Apple added for the UI font, which I presume is part of the reason for the custom spacing.
Is there a way to say "Not that Helvetica Neue, the other one," ideally via something like CSS?
A related question that would solve the same problem: Is there a way to specify "System UI Font" in CSS, that would match the system (HN++ on Yosemite, Lucida Grande on earlier systems)?