If I try making a bitmap of a NSButton using the following code:
NSBitmapImageRep *rep = [button bitmapImageRepForCachingDisplayInRect:button.bounds];
[button cacheDisplayInRect:button.bounds toBitmapImageRep:rep];
NSData *tiffData = rep.TIFFRepresentation;
I get a blank image if the NSButton has its bezelStyle set to NSBezelStyleGlass.
If I change the bezel style to something else like NSBezelStyleFlexiblePush I do get an image