How to NOT change the background when NSButton clicked

How to NOT change the background when NSButton clicked

I have a NSButton on a view I subclass the NSView and draw something in
the drawRect:, and the view is belong to a NSWindow which

Then when I click the button, I find the view become transparent, which I
guess is because the button click changes the button's background. So I
subclass the NSButton,

- (void)mouseDown:(NSEvent *)theEvent {
[self.window.contentView setNeedsDisplay:YES];
[super mouseDown:theEvent];
[self.window.contentView setNeedsDisplay:YES];
}
Right now when I mouseDown and mouseUp, the view seems right, except there
is a flash transparent when I click, even more, when I mouseDown and move
out of the button, the background becomes transparent again.


Can anyone tell me what should I do to make the background of a clicked
button do not change?
or where to put the redraw code.
THANKS!