[view validateScrollers];

to your scaleImageTo: method.

so it basically looks like:
- (void)scaleImageTo:(float)_scale
{
if(_scale>0)
scale=_scale;

[view scaleFrameBy:scale];
[zoomControl setFloatValue:(scale*100)];
[view validateScrollers];
}

it works great for me.
stupidFish23