NSScrollViewの内容センタリング ref: http://qiita.com/1024jp/items/f3cd7bc7765cf090fa31

MyCenteringClipView.h

#import <Cocoa/Cocoa.h>

@interface MyCenteringClipView : NSClipView
@end

MyCenteringClipView.m

#import "MyCenteringClipView.h"

@implementation MyCenteringClipView

- (NSRect)constrainBoundsRect:(NSRect)proposedBounds
{
    NSRect documentBounds = [[self documentView] bounds];
    NSPoint delta         = NSMakePoint(NSWidth(documentBounds)  - NSWidth(proposedBounds),
                                        NSHeight(documentBounds) - NSHeight(proposedBounds));

    CGFloat x = (delta.x < 0) ? (delta.x / 2) : MAX(0, MIN(proposedBounds.origin.x, delta.x));
    CGFloat y = (delta.y < 0) ? (delta.y / 2) : MAX(0, MIN(proposedBounds.origin.y, delta.y));
    proposedBounds.origin = NSMakePoint(floor(x), floor(y));

    return proposedBounds;
}

@end