Resize custom NSView inside NSScrollView by setFrameSize triggers drawRect
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So I have this basic custom NSView / NSScrollView setup, constructed in awakeFromNib programmatically, exactly like in Apple's ScrollView Programming guide.

My custom view initially has a certain frame-size and the scroll-position remains at the origin of the document-view (that is my custom-view).

Now, from a different thread, by issueing a performSelectorOnMainThread, I change the framesize of the custom-view by setFrameSize(newSize), only making it bigger in height.
Origin and width stay the same.

But now I am getting drawRect: invoked, with a dirty rect of (0, 0, viewwidth, viewheight), which I think is strange, at least it is not optimal because there is a redraw-request in an area where nothing has changed.

From the Referencepage of NSView, I assumed that setFrameSize: does not cause drawRect: to be triggered,  long as you don't issue a setNeedsDrawInRect: or similar.

So, does anyone know, why, in the case of being contained in a NSScrollView, this drawRect: call is made by Cocoa ?
And which object (NSScrollView, NSClipView, NSNotificationCenter...) is issueing this call ?

Any hint greatly appreciated, 
Joerg
