[cssom-view] scope of the 'scroll-behavior' property

I'm a little concerned about the scope of the 'scroll-behavior'
property defined in
http://dev.w3.org/csswg/cssom-view/#smooth-scrolling:-the-%27scroll-behavior%27-property

It seems like this starts from the reasonable premise of
categorizing scrolling into three groups:

 (1) scrolling done by a user scrolling action (which is
     intentionally not influenced by any mechanisms in the spec, but
     controlled by the OS defaults and user settings)

 (2) scrolling that happens from navigation (e.g., navigating to a
     named anchor)

 (3) scrolling that happens through scrolling APIs.

This spec adds ScrollOptions parameters to a number of APIs (such as
window.scroll, window.scrollBy, window.scrollTo,
element.scrollIntoView, element.scrollTop, element.scrollLeft) to
address case (3): http://dev.w3.org/csswg/cssom-view/#scrolloptions

The scroll-behavior property, on the other hand, says it covers both
(2) and (3).

First, the spec doesn't seem to say how the two interact:  for the
methods that have ScrollOptions parameters, which of the
'scroll-behavior' or ScrollOptions wins?

I tend to think that I'd prefer the answer that 'scroll-behavior'
simply doesn't affect case (3); thus an author changing
'scroll-behavior' in order to affect case (2) doesn't end up
accidentally affecting case (3) as well.  It seems odd to have an
API whose default behavior is changed by a CSS property but that can
override the behavior -- the CSS property in question feels not very
different from a global variable on the window object.

On the other hand, I could imagine an author using 'instant' because
they want a particular container that contains discrete items never
to scroll smoothly.  But that case seems better handled by scroll
snapping, since it should also affect case (1).

Maybe there's some more useful interaction between the two ways of
specifying the scroll behavior, though?

(Also, that's a horrible named anchor at the start of this message!)

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)