[cssom-view] Tweak the check for 'is body scrollable?' to better match Gecko. Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=28487

Author: zcorpan

cssom-view/Overview.bs

@@ -81,6 +81,8 @@ spec:css-display-3; type:value; for:display; text:table
 spec:css-position-3; type:property; text:position
 </pre>
 
+<script src=https://resources.whatwg.org/file-bug.js async></script>
+
 Background {#background}
 ========================
 
@@ -115,6 +117,16 @@ HTML.
 Elements and <a>viewports</a> have an associated <dfn>scrolling box</dfn> if has a scrolling mechanism<!--XXX xref--> or it overflows
 its content area<!--XXX xref--> and the used value of the 'overflow-x' or 'overflow-y' property is ''overflow/hidden''. [[!CSS3-BOX]]
 
+An element is <dfn>potentially scrollable</dfn> if all of the following conditions are true:
+
+* The element has an associated <a>CSS layout box</a>.
+* The element is not <a>the HTML <code>body</code> element</a>, or it is and
+  the root element's used value of the 'overflow-x' or 'overflow-y' properties is not ''overflow/visible''.
+* The element's used value of the 'overflow-x' or 'overflow-y' properties is not ''overflow/visible''.
+
+Note: An element that is <a>potentially scrollable</a> might not have a <a>scrolling box</a>.
+For instance, it could have 'overflow' set to ''overflow/auto'' but not have its content overflowing its content area.
+
 A <a>scrolling box</a> of a <a>viewport</a> or element has two <dfn>overflow directions</dfn>, depending on the <a>viewport’s</a> or
 element's <a>block flow direction</a> and <a>inline base direction</a>, as follows:
 
@@ -922,7 +934,7 @@ revision of CSS or HTML.
 <p class=atrisk>The <dfn attribute for=Document>scrollingElement</dfn> attribute, on getting, must run these steps:
 
 1. If the {{Document}} is in <a>quirks mode</a>, follow these substeps:
-    1. If <a>the HTML <code>body</code> element</a> exists, and it does not have an associated <a>scrolling box</a>, return <a>the HTML <code>body</code> element</a> and abort these steps.
+    1. If <a>the HTML <code>body</code> element</a> exists, and it is not <a>potentially scrollable</a>, return <a>the HTML <code>body</code> element</a> and abort these steps.
     1. Return null and abort these steps.
 1. If there is a root element, return the root element and abort these steps.
 1. Return null.
@@ -1062,7 +1074,7 @@ The <dfn method for=Element lt="scroll(options)|scroll(x, y)">scroll()</dfn> met
 	and terminate these steps.
 1. If the element is <a>the HTML <code>body</code> element</a>,
 	the {{Document}} is in <a>quirks mode</a>,
-	and the element does not have an associated <a>scrolling box</a>,
+	and the element is not <a>potentially scrollable</a>,
 	invoke {{Window/scroll()}} with <var>options</var> as the only argument,
 	and terminate these steps.
 1. If the element does not have any associated <a>CSS layout box</a>,
@@ -1095,7 +1107,7 @@ The <dfn attribute for=Element>scrollTop</dfn> attribute, on getting, must retur
 
 1. If the element is the root element and the {{Document}} is in <a>quirks mode</a>, return zero and terminate these steps.
 1. If the element is the root element return the value of {{Window/scrollY}}.
-1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element does not have an associated <a>scrolling box</a>, return the value of {{Window/scrollY}}.
+1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element is not <a>potentially scrollable</a>, return the value of {{Window/scrollY}}.
 1. If the element does not have any associated <a>CSS layout box</a>, return zero and terminate these steps.
 1. Return the y-coordinate of the <a>scrolling area</a> at the alignment point with the top of the <a>padding edge</a> of the element.
 
@@ -1105,15 +1117,15 @@ When setting the {{Element/scrollTop}} attribute these steps must be run:
 1. <a>Normalize non-finite values</a> for <var>y</var>.
 1. If the element is the root element and the {{Document}} is in <a>quirks mode</a>, terminate these steps.
 1. If the element is the root element invoke {{Window/scroll()}} with {{Window/scrollX}} as first argument and <var>y</var> as second argument, and terminate these steps.
-1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element does not have an associated <a>scrolling box</a>, invoke {{Window/scroll()}} with {{Window/scrollX}} as first argument and <var>y</var> as second argument, and terminate these steps.
+1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element is not <a>potentially scrollable</a>, invoke {{Window/scroll()}} with {{Window/scrollX}} as first argument and <var>y</var> as second argument, and terminate these steps.
 1. If the element does not have any associated <a>CSS layout box</a>, the element has no associated <a>scrolling box</a>, or the element has no overflow, terminate these steps.
 1. <a lt='scroll an element'>Scroll the element</a> to {{Element/scrollLeft}},<var>y</var>, with the scroll behavior being <code>"auto"</code>.
 
 The <dfn attribute for=Element>scrollLeft</dfn> attribute, on getting, must return the result of running these steps:
 
 1. If the element is the root element and the {{Document}} is in <a>quirks mode</a>, return zero and terminate these steps.
 1. If the element is the root element return the value of {{Window/scrollX}}.
-1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element does not have an associated <a>scrolling box</a>, return the value of {{Window/scrollX}}.
+1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element is not <a>potentially scrollable</a>, return the value of {{Window/scrollX}}.
 1. If the element does not have any associated <a>CSS layout box</a>, return zero and terminate these steps.
 1. Return the x-coordinate of the <a>scrolling area</a> at the alignment point with the left of the <a>padding edge</a> of the element.
 
@@ -1123,23 +1135,23 @@ When setting the {{Element/scrollLeft}} attribute these steps must be run:
 1. <a>Normalize non-finite values</a> for <var>x</var>.
 1. If the element is the root element and the {{Document}} is in <a>quirks mode</a>, terminate these steps.
 1. If the element is the root element invoke {{Window/scroll()}} with <var>x</var> as first argument and {{Window/scrollY}} as second argument, and terminate these steps.
-1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element does not have an associated <a>scrolling box</a>, invoke {{Window/scroll()}} with <var>x</var> as first argument and {{Window/scrollY}} as second argument, and terminate these steps.
+1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a>, and the element is not <a>potentially scrollable</a>, invoke {{Window/scroll()}} with <var>x</var> as first argument and {{Window/scrollY}} as second argument, and terminate these steps.
 1. If the element does not have any associated <a>CSS layout box</a>, the element has no associated <a>scrolling box</a>, or the element has no overflow, terminate these steps.
 1. <a lt='scroll an element'>Scroll the element</a> to <var>x</var>,{{Element/scrollTop}}, with the scroll behavior being <code>"auto"</code>.
 
 The <dfn attribute for=Element>scrollWidth</dfn> attribute must return the result of running these steps:
 
 1. Let <var>viewport width</var> be the width of the <a>viewport</a> excluding the width of the scroll bar, if any, or zero if there is no <a>viewport</a>.
 1. If the element is the root element and the {{Document}} is not in <a>quirks mode</a> return max(<a>viewport</a> <a>scrolling area</a> width, <var>viewport width</var>).
-1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a> and the element has no associated <a>scrolling box</a>, return max(<a>viewport</a> <a>scrolling area</a> width, <var>viewport width</var>).
+1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a> and the element is not <a>potentially scrollable</a>, return max(<a>viewport</a> <a>scrolling area</a> width, <var>viewport width</var>).
 1. If the element does not have any associated <a>CSS layout box</a> return zero and terminate these steps.
 1. Return the width of the element's <a>scrolling area</a>.
 
 The <dfn attribute for=Element>scrollHeight</dfn> attribute must return the result of running these steps:
 
 1. Let <var>viewport height</var> be the height of the <a>viewport</a> excluding the height of the scroll bar, if any, or zero if there is no <a>viewport</a>.
 1. If the element is the root element and the {{Document}} is not in <a>quirks mode</a> return max(<a>viewport</a> <a>scrolling area</a> height, <var>viewport height</var>).
-1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a> and the element has no associated <a>scrolling box</a>, return max(<a>viewport</a> <a>scrolling area</a> height, <var>viewport height</var>).
+1. If the element is <a>the HTML <code>body</code> element</a>, the {{Document}} is in <a>quirks mode</a> and the element is not <a>potentially scrollable</a>, return max(<a>viewport</a> <a>scrolling area</a> height, <var>viewport height</var>).
 1. If the element does not have any associated <a>CSS layout box</a> return zero and terminate these steps.
 1. Return the height of the element's <a>scrolling area</a>.
 

cssom/Overview.bs

@@ -116,6 +116,8 @@ spec:css-position-3; type:property; text:left
 spec:html5; type:element; text:style
 </pre>
 
+<script src=https://resources.whatwg.org/file-bug.js async></script>
+
 Introduction {#introduction}
 ============================