how to stop NSScrollView from scrolling on resize?

Question

I'm writing a PDFViewer for a 3rd party PDF library. I have a subclass of NSScrollView and a subclass of NSView. The problem is the NSScrollView will scroll up and down when I resize window.

here is the relevant code:

whenever windows is resized, my subclass of NSScrollView receive this message:

-(void)handleFrameChange:(NSNotification*)notification {
    id * docView = [self documentView];
    [docView windowResized];
}

in my docView, a subclass of NSView, containing code to draw pdf

- (void)windowResized;
{
    NSSize size = [[self superview] bounds].size;
    int width = (int) size.width;
    int height = (int) size.height;
    _pdfView->OnSize(width, height);

    double w = _pdfView->GetCanvasWidth();
    double h = _pdfView->GetCanvasHeight();
    [self setFrameSize:NSMakeSize(w, h)];

    //allocate buffer
    _buf.resize(width * height * 4, NULL);

    [self setNeedsDisplay:YES];
 }