How do I use relativesource if I don't know the ancestorlevel?

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Binding

I'm puzzled by the Binding RelativeSource functionality. My question is in several parts:

I want to bind to the data context of the whole window from a control buried deep in the visual tree

1. How do I determine the AncestorLevel?

2. Why should I need to specify this at all when I know there is only ever one window?

3. If I do figure out the AncestorLevel and get it working, doesn't this make the code brittle? If I change the nesting of the controls, it will break.

Any advice would be gratefully received.

What I have tried:

I have tried simply omitting the AncestorLevel. I thought this would then simply travel towards the root of the visual tree and find the first control with the matching type. I have seen examples on the internet that seem to work this way, and I could have sworn I used this behaviour some years ago. It doesn't work now. Did it ever work or am I just imagining that?