To clarify, if I have overflow-anchor: none on scroller x, and overflow-anchor:visible on an element y within the scroller, this would mean the following things right?
- Scroll anchoring is disabled for scroller x
- All elements between x and y will not be used as anchors, but all of y's children will still be potential anchor candidates?

I worry that authors may disable scroll anchoring on a nested scroller, without realizing that all of the scrollers' children are opted out from being anchor candidates for the main scroller.

No, I think if findAnchor hits an element with overflow-anchor:none it should skip over it and not descend into it.

There's no way to exclude an element from an inner scroller but not an outer scroller.  But I'm not sure when that would be useful?

For nested scrollers, we have the following scenarios: 

Want anchoring on outer and inner: do nothing
Want anchoring on neither: overflow-anchor: none on inner and outer
Want anchoring on inner not on outer: overflow-anchor:none on outer
Want anchoring on outer not on inner: 
  - overflow-anchor: none on inner scroller if also want to exclude inner from outer's anchor selection, else
  - wrap contents of inner scroller within a div and overflow-anchor: none on the div

In the last case, the author can’t specify to anchor to things inside the inner scroller for the outer scroller, but presumably this is not really useful.

Tab, do you have opinions on this? The more I think about it, the more it feels weird to me to have one property control both whether you do anchoring and whether you can be an anchor. Using wrapper nodes to solve problems also is less than ideal since we're trying to push authors to use fewer DOM nodes to get better performance.

I don't think it's weird at first glance, but this is also getting to the point that we should be WICG'ing it and getting other people to look at it. ^_^

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/82a6ae9efa7fa7f60d9af62bd2b3f2ebe023e177

commit 82a6ae9efa7fa7f60d9af62bd2b3f2ebe023e177
Author: ymalik <ymalik@chromium.org>
Date: Mon Aug 15 20:32:48 2016

Exclude elements with overflow-anchor:none from anchor node selection

BUG= 635716 

Review-Url: https://codereview.chromium.org/2246803002
Cr-Commit-Position: refs/heads/master@{#412035}

[modify] https://crrev.com/82a6ae9efa7fa7f60d9af62bd2b3f2ebe023e177/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
[modify] https://crrev.com/82a6ae9efa7fa7f60d9af62bd2b3f2ebe023e177/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp

I've added the questions around the shape of this API to the open issues section of the explainer doc and invited feedback on https://github.com/WICG/interventions/issues/2.  Let's close this and open new bugs for any future API changes we decide to make.