In AS3: Does masking an object make the mask child of the masked object?

Say I have a background which has an event listener for the browser resizing, for obvious purpose, to match the browser size.

And I have a mask, masking this backgorund.

I figured out that when the background is resized, mask starts to resize, but it is not even the child of the background.

But what I want is a non-resizing mask…

So I need to cut the funicle between mask and the background. I need this mask to mask the background but don’t want to scale with the background… How will I do it? A probable answer will be: a child free of its parent. Any comments?