[css-scroll-snap] Add scrolling types section.

Author: fantasai

css-scroll-snap/Overview.bs

@@ -667,6 +667,83 @@ Snapping Mechanics {#snap-concepts}
     to ensure a minimum reasonable behavior that authors can depend on
     when designing their interfaces with scroll-snapping in mind.
 
+
+<!--
+████████ ██    ██ ████████  ████████  ██████
+   ██     ██  ██  ██     ██ ██       ██    ██
+   ██      ████   ██     ██ ██       ██
+   ██       ██    ████████  ██████    ██████
+   ██       ██    ██        ██             ██
+   ██       ██    ██        ██       ██    ██
+   ██       ██    ██        ████████  ██████
+-->
+
+Types of Scrolling Methods {#scroll-types}
+------------------------------------------
+
+    When a page is scrolled,
+    the action is performed with
+    an intended end position
+    and/or an intended direction.
+    Each combination of these two things
+    defines a distinct category of scrolling,
+    which can be treated slightly differently
+
+    : <dfn export local-lt="explicit" lt="explicit scroll">explicit scrolling</dfn>
+    :: A scroll is <a>explicit</a>
+        if it has an intended end position,
+        but no intended direction.
+        <div class="example">
+            Common examples include methods such as:
+
+            * a panning gesture,
+                released without momentum
+            * manipulating the scrollbar "thumb" explicitly
+            * programmatically scrolling via APIs such as {{Window/scrollTo()}}
+            * tabbing through the document's focusable elements
+            * navigating to an anchor within the page
+        </div>
+
+    : <dfn export local-lt="distance-based" lt="distance scroll">distance scrolling</dfn>
+    :: A scroll is <a>distance-based</a>
+        if it has both an intended end position
+        and an intended direction.
+
+        <div class="example">
+            Common examples include:
+
+            * a "fling" gesture,
+                interpreted with momentum
+            * programmatically scrolling via APIs such as {{Window/scrollBy()}}
+        </div>
+
+        This category includes <dfn lt="inertial scroll|inertial">inertial scrolling</dfn>,
+        which are scroll gestures interpreted with momentum
+        that carries the scroll beyond user's initial gesture,
+        whose “intended” end position might be implicitly determined by the UA's scrolling physics,
+        but whose effective distance is derived from the strength of the user's fling.
+        The scroll position that an <a>inertial</a> scroll would naturally land on
+        without further intervention is its <dfn noexport>natural end-point</dfn>.
+
+    : <dfn export local-lt="directional" lt="directional scroll">directional scrolling</dfn>
+    :: A scroll is <a>directional</a> if it has an intended direction,
+        but no intended end point.
+        <div class="example">
+            Common examples include:
+
+            * pressing an arrow key on the keyboard
+            * a swiping gesture interpreted as a fixed (rather than inertial) scroll
+        </div>
+
+    Additionally, because page layouts usually align things vertically and/or horizontally,
+    UAs sometimes <dfn export>axis-lock</dfn> a scroll when its direction
+    is sufficiently vertical or horizontal.
+    An <a>axis-locked</a> scroll is bound to only scroll along that axis.
+    This prevents,
+    for example,
+    a <em>nearly</em> horizontal fling gesture from gradually drifting up or down as well,
+    because it is very difficult to fling in a precisely horizontal line.
+
 <!--
    ██   ████████         ███████  ████████
  ████   ██     ██       ██     ██ ██     ██