NSScrollview and transparent, overlay NSScroller subclasses

I have made a slick NSScroller subclass, but can't figure out how to make it overlay on top of the NSScrollView instead of pushing the documentView aside.

Here you can see the background of a NSCollectionView that I wish to make 100% wide, and have the scroller sit along top. Currently, I have to set a white background to the scroller because drawing with a clearColor is not showing as transparent, but as black.

Am I going about this the wrong way? Am I missing something obvious here? How can I achieve the behavior of a transparent-tracked NSScroller that sits atop a NSScrollView's contents?

I was able to get the positioning by implementing tile in the subclass OF NSSCROLLVIEW

- (void)tile {
    [super tile];
    [[self contentView] setFrame:[self bounds]];
}

And it turns out that trying to draw clear color was the problem to begin with. In the scroller subclass, I just omitted any drawing that had to do with the slider track completely BY OVERRIDING DRAWRECT: OF THE NSSCROLLER SUBCLASS, LIKE SO:

- (void)drawRect:(NSRect)dirtyRect
{
    [self drawKnob];
}