so you have a NSButton ... subclass THAT so you attach a view:
@interface ButtonWithWebViewOnHover : NSButton
@property(strong) WebView *webView;
@end
override mouseEntered and mouseExited there and toggle hidden
...... wait.... we seem to be reinventing the wheel
use NSPopover (from apple directly, but not as graphically flexible as the next:)
or MAAttachedWindow (http://mattgemmell.com/2007/10/03/maattachedwindow-nswindow-subclass/)