[css-snappoints] Various issues

[Many of these issues are derived from
https://wiki.mozilla.org/index.php?title=Gecko:CSSScrollSnapping.]

Can we change the name of the spec to css-scrollsnap? We don't snap to
points, but to edges.

For that reason, I think scroll-snap-points-x/y should be renamed to
scroll-snap-positions-x/y.

scroll-snap-type seems unnecessarily verbose. I suggest we just use
scroll-snap.

I think independent control over vertical and horizontal snapping will be
very useful. (E.g, vertical scrolling through a list of items, some of
which may overflow horizontally.) I propose making scroll-snap a shorthand
for scroll-snap-x and scroll-snap-y.

What should happen when we're currently snapped to an edge and that edge
moves? (E.g. due to dynamic changes to the scroll-snap-positions-x/y
properties, or due to a layout change moving an edge induced by an element
with scroll-snap-edge?) In proximity mode I guess it's OK to just do
nothing. But in mandatory mode, I think we should re-snap to the nearest
snapping edge, or something like that.

I have a proposal for a scroll-snap-edge property that adds edges of an
element's margin-box or border-box to the list of allowable snapping
positions. I think in most cases this is simpler and more robust than
scroll-snap-positions-x/y. I would like this, or some version of it, to be
incorporated into the spec. The details are here:
https://wiki.mozilla.org/index.php?title=Gecko:CSSScrollSnapping

For scroll-snap-edge to be useful, we need scroll-snap-positions-x/y to
support a "none" value, which should be the initial value.

If we do that, then we need to specify what happens when there is mandatory
snapping but no snapping positions. In my linked proposal, I have suggested
we snap to 0 in that specific case. (As I noted in an earlier email, there
are use-cases that require the set of allowed snapping positions to exclude
0.)

Rob
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