Cocoa_GetDisplayUsableBounds is incorrectly calculating the usable bounds.  [NSScreen visibleFrame] returns a rect in global screen coords (same as [NSScreen frame], not coords relative to the origin of the particular screen.  Also the rect's origin needs to be converted to SDL's top-left convention instead of Cocoa's bottom-left.

This:
    const CGRect cgrect = CGDisplayBounds(cgdisplay);
    const NSRect frame = [screen visibleFrame];
    // !!! FIXME: I assume -[NSScreen visibleFrame] is relative to the origin of the screen in question and not the whole desktop.
    // !!! FIXME: The math vs CGDisplayBounds might be incorrect if that's not the case, though. Check this.
    rect->x = (int)(cgrect.origin.x + frame.origin.x);
    rect->y = (int)(cgrect.origin.y + frame.origin.y);
    rect->w = (int)frame.size.width;
    rect->h = (int)frame.size.height;

Should be:
    const NSRect frame = [screen visibleFrame];
    rect->x = (int)frame.origin.x;
    rect->y = (int)(CGDisplayPixelsHigh(kCGDirectMainDisplay) - frame.origin.y - frame.size.height);
    rect->w = (int)frame.size.width;
    rect->h = (int)frame.size.height;