[css-scroll-snap] Add “Choosing Snap Positions” section.

Author: fantasai

css-scroll-snap/Overview.bs

@@ -649,6 +649,24 @@ Scroll Snap Limits: the 'scroll-snap-stop' property {#scroll-snap-stop}
 
     Issue: It's been proposed to rename ''normal'', but not to what.
 
+Snapping Mechanics {#snap-concepts}
+===================================
+
+    The precise model algorithm to select a <a>snap position</a> to snap to
+    is intentionally left mostly undefined,
+    so that user agents can take into account sophisticated models of user intention and interaction
+    and adjust how they respond over time,
+    to best serve the user.
+
+    This section defines some useful concepts to aid in discussing scroll-snapping mechanics,
+    and provides some guidelines for what an effective scroll-snapping strategy might look like.
+    User agents are encouraged to adapt this guidance
+    and apply their own best judgement
+    when defining their own snapping behavior.
+    It also provides a small number of behavior requirements,
+    to ensure a minimum reasonable behavior that authors can depend on
+    when designing their interfaces with scroll-snapping in mind.
+
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@@ -704,6 +722,76 @@ Axis vs Point-Snapping {#snap-dimensions}
         while the vertical was aligned to a different element
         (which is the behavior you'd get if the <a>scroll container</a> was <a>axis-snapping</a>).
 
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+
+Choosing Snap Positions {#choosing}
+-----------------------------------
+
+    A <a>scroll container</a> can have many <a>snap areas</a>
+    scattered throughout its <a>scrollable overflow region</a>.
+    A naive algorithm for selecting a <a>snap position</a>
+    can produce behavior that is unintuitive for users,
+    so care is required when designing a selection algorithm.
+    Here are a few pointers that can aid in the selection process:
+
+    * <a>Snap positions</a> should be chosen to minimize the distance between the end-point
+        (or the <a>natural end-point</a>)
+        and the final snapped scroll position,
+        subject to the additional constraints listed in this section.
+
+    * <a>Point-snapping</a> is all-or-nothing;
+        if the <a>snap position</a> of an element is chosen to align to,
+        the <a>scroll container</a> must set its scroll position
+        according to the element's <a>snap positions</a> in <em>both</em> axises;
+        the <a>scroll container</a> <em>must not</em> “partially align” to the element
+        by taking its <a>snap position</a> in one axis
+        and aligning the other axis according to something else.
+
+    * If a scroll is <a>axis-locked</a> and the <a>scroll container</a> is <a>axis-snapping</a>,
+        any <a>snap positions</a> in the other axis should be ignored
+        during the scroll.
+        (However, <a>snap positions</a> in the other axis can still effect the final scroll position.)
+
+        If a scroll is <a>axis-locked</a> and the <a>scroll container</a> is <a>point-snapping</a>,
+        <a>snap positions</a> should be penalized in the selection process
+        according to the amount of other-axis scrolling they would cause.
+
+    * <a>Snap positions</a> should be ignored if their elements are far outside of the “corridor”
+        that the <a>snapport</a> defines as it moves through the <a>scrollable overflow region</a>
+        during an <a>inertial scroll</a>,
+        or a hypothetical “corridor” in the direction of a <a>directional scroll</a>,
+        or the <a>snapport</a> after an <a>explicit scroll</a>.
+        (This is to prevent a far-offscreen element
+        from having difficult-to-understand effects
+        on the scroll position.)
+
+    * User agents <em>must</em> ensure that a user can “escape” a <a>snap position</a>,
+        regardless of the scroll method.
+        For example, if the snap type is ''mandatory''
+        and the next <a>snap position</a> is more than two screen-widths away,
+        a naïve “always snap to nearest” selection algorithm would “trap” the user
+        if they were panning with a touch gesture;
+        a sufficiently large distance would even trap fling scrolling!
+        Instead, a smarter algorithm that only returned to the starting <a>snap position</a>
+        if the end-point was a fairly small distance from it,
+        and otherwise ignored the starting snap position,
+        would give better behavior.
+
+        (This implies that a <a>directional scroll</a> must always ignore the starting <a>snap positions</a>.)
+
+    * If a page is navigated to a fragment that defines a target element
+        (one that would be matched by '':target''),
+        and that element defines some <a>snap positions</a>,
+        the user agent should <a>snap</a> to one of that element's <a>snap positions</a>.
+        The user agent may do this even when the <a>scroll container</a> has ''scroll-snap-type: none''.
 
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