[css-overscroll-behavior-1] Add logical longhands #2473

Add logical longhands that map to corresponding physical one.

To do this properly introduce dfn for "overscroll behavior".
The physical longhands refer to this definition while the shorthand
and logical longhands refer to the physical longhands.

Other fixes:
 - Drop unnecessary overview section
 - Use 'Animation type' instead of animatable
 - Correct wrapping

Author: majido

css-overscroll-behavior-1/Overview.bs

@@ -9,7 +9,8 @@ URL: https://drafts.csswg.org/css-overscroll-behavior-1/
 Editor: Benoit Girard, Facebook, bgirard@fb.com
 Editor: Majid Valipour, Google, majidvp@google.com, w3cid 81464
 
-Abstract: This module defines 'overscroll-behavior' to control the behavior when the scroll position of a <a>scroll container</a> reaches the edge of the <a>scrollport</a>.
+Abstract: This module defines 'overscroll-behavior' to control the behavior when the scroll position
+Abstract: of a <a>scroll container</a> reaches the edge of the <a>scrollport</a>.
 Abstract: This allows content authors to hint that the <a>boundary default actions</a>,
 Abstract: such as scroll chaining and overscroll, should not be triggered.
 </pre>
@@ -61,7 +62,7 @@ shortcoming.
 
 
 Motivating Examples {#motivating-examples}
-=================
+==========================================
 
 
 <div class=example>
@@ -110,7 +111,7 @@ scroller to prevent both scroll chaining and overscroll affordance.
 </div>
 
 Scroll chaining and boundary default actions {#scroll-chaining-and-boundary-default-actions}
-==========================
+============================================================================================
 
 <em>Operating Systems have rules for scrolling such as scroll chaining and overscroll affordances.
 This specification does not mandate if and how scroll chaining or overscroll affordances be
@@ -138,43 +139,25 @@ interacting with the page, for example displaying a overscroll UI affordance. Co
 >non-local boundary default action</dfn> interacts with the page, for example scroll chaining or a
 navigation action.
 
-Overview {#overview}
-==========================
-
-This module introduces control over the behavior of a <a>scroll container</a> element when its
-<a>scrollport</a> reaches the boundary of its scroll box. It allows the content author to specify
-that a <a>scroll container</a> element must prevent scroll chaining and/or overscroll affordances.
 
 Overscroll Behavior Properties {#overscroll-behavior-properties}
-==========================
+================================================================
+
+The <dfn>overscroll behavior</dfn> controls the permitted <a>boundary default action</a> for a
+<a>scroll container</a> element when its <a>scrollport</a> reaches the boundary of its scroll box.
+
 
-These properties specify how a <a>scroll container</a> element must behave when scrolling. A element
-that is not <a>scroll container</a> must accept but ignore the values of this property. This
-property must be applied to all input methods supported by the user agent.
+The 'overscroll-behavior' property specifies the <a>overscroll behavior</a> for a <a>scroll
+container</a> element. It allows the content author to specify that a <a>scroll container</a>
+element must prevent scroll chaining and/or overscroll affordances.
+
+An element that is not <a>scroll container</a> must accept but ignore the values of this property.
+This property must be applied to all scrolling methods supported by the user agent.
 
 Note: This property should provide guarantees that are, at least, as strong as <a>preventDefault</a>
 for preventing both scroll chaining and overscroll. Doing otherwise would cause content authors to
 use <a>preventDefault</a> instead.
 
-<pre class=propdef>
-Name: overscroll-behavior-x, overscroll-behavior-y
-Value: contain | none | auto
-Initial: auto
-Applies to: <a>scroll container</a> elements
-Inherited: no
-Percentages: N/A
-Media: visual
-Computed value: as specified
-Animatable: no
-Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
-</pre>
-
-The 'overscroll-behavior-x' property specifies the behavior of the 'overscroll-behavior'
-in the horizontal direction and the 'overscroll-behavior-y' property specifies the handling of
-the 'overscroll-behavior' in the vertical direction. When scrolling is performed along both the
-horizontal and vertical axes at the same time, the 'overscroll-behavior' of each respective
-axis should be considered independently.
-
 <pre class=propdef>
 Name: overscroll-behavior
 Value: [ contain | none | auto ]{1,2}
@@ -183,22 +166,25 @@ Applies to: <a>scroll container</a> elements
 Inherited: no
 Media: visual
 Computed value: see individual properties
-Animatable: no
+Animation type:: discrete
 Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
 </pre>
 
-The two values specify the behavior in the horizontal and vertical direction, respectively. If only one value is specified, the second value defaults to the same value.
+
+The 'overscroll-behavior' property is a shorthand property that sets the specified values of
+'overscroll-behavior-x' and 'overscroll-behavior-y' in that order. If only one value is specified,
+the second value defaults to the same value.
 
 Values have the following meanings:
 
-<dl dfn-for="overscroll-behavior, overscroll-behavior-x, overscroll-behavior-y" dfn-type="value">
+<dl dfn-for="overscroll-behavior, overscroll-behavior-x, overscroll-behavior-y, overscroll-behavior-inline, overscroll-behavior-block" dfn-type="value">
   <dt><dfn>contain</dfn>
   <dd>
-    This value indicates that the element must not perform <a>non-local boundary default actions</a>
+    This value indicates that the element must not perform non-<a>local boundary default actions</a>
     such as scroll chaining or navigation. The user agent must not perform scroll chaining to any
     ancestors along the <a>scroll chain</a> regardless of whether the scroll originated at this
     element or one of its descendants. This value must not modify the behavior of how <a>local
-    boundary default actions</a> should behave, such as overscroll behavior.
+    boundary default actions</a> should behave, such as showing any overscroll affordances.
   <dt><dfn>none</dfn>
   <dd>
     This value implies the same behavior as <a value for=overscroll-behavior>contain</a> and in
@@ -216,9 +202,51 @@ these values will have no side effects for a compliant implementation.
 Note: Programmatic scrolling is clamped and can not trigger any <a>boundary default actions</a>.
 
 
+Physical Longhands for 'overscroll-behavior'  {#overscroll-behavior-longhands-physical}
+---------------------------------------------------------------------------------------
+
+<pre class=propdef>
+Name: overscroll-behavior-x, overscroll-behavior-y
+Value: contain | none | auto
+Initial: auto
+Applies to: <a>scroll container</a> elements
+Inherited: no
+Percentages: N/A
+Media: visual
+Computed value: as specified
+Animation type:: discrete
+Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
+</pre>
+
+The 'overscroll-behavior-x' property specifies the <a>overscroll behavior</a> in the horizontal
+axis and the 'overscroll-behavior-y' property specifies the <a>overscroll behavior</a> in the
+vertical axis. When scrolling is performed along both the horizontal and vertical axes at the
+same time, the <a>overscroll behavior</a> of each respective axis should be considered
+independently.
+
+
+Flow-relative Longhands for 'overscroll-behavior'  {#overscroll-behavior-longhands-logical}
+-------------------------------------------------------------------------------------------
+
+<pre class=propdef>
+Name: overscroll-behavior-inline, overscroll-behavior-block
+Value: contain | none | auto
+Initial: auto
+Applies to: <a>scroll container</a> elements
+Inherited: no
+Percentages: N/A
+Media: visual
+Computed value: as specified
+Animation type:: discrete
+Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
+</pre>
+
+These properties correspond to the 'overscroll-behavior-x' and 'overscroll-behavior-y' properties.
+The mapping depends on the element's 'writing-mode'.
+
 
 Security and Privacy Considerations {#security-and-privacy}
-===================================
+===========================================================
 There are no known security or privacy impacts of this feature. The feature may be used to prevent
 certain native UI features such as overscroll affordances and overscroll navigations (e.g., pull-
 to-refresh, swipe navigations). However, this does not expose any additional abilities beyond what