Rewritten Answer
I'm not seeing an NSTexturedSquareBezelStyle button showing up borderless in Yosemite:
Image may be NSFW.
Clik here to view.
This is a stock button dragged from the palette in Interface Builder. I encourage you to post your code since it's likely you're generating your button in code. Here's my own code for generating the same button:
NSButton * anotherButton = [[NSButton alloc] initWithFrame:NSMakeRect(10.0, 10.0, 100.0, 100.0)];
anotherButton.bezelStyle = NSTexturedSquareBezelStyle;
anotherButton.title = @"Line 1\nLine 2";
Proof:
Image may be NSFW.
Clik here to view.
If you're seeing different results under Yosemite, you'll need to post your code. At a guess, you might be initializing your button with -init instead of -initWithFrame:, which can cause all kinds of drawing issues since NSButton is an NSView and therefore its designated initializer is -initWithFrame:. Just a guess though.
