[css-overflow] Define scrollport as a term.

Author: fantasai

css-overflow-3/Overview.bs

@@ -287,9 +287,13 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 	(including any <a>ink overflow</a>)
 	is clipped to its padding edge,
 	and if so,
-	whether it is a <dfn export>scrollable box</dfn>
+	whether it is a <dfn export>scroll container</dfn>
 	that allows the user to scroll clipped parts of its <a>scrollable overflow region</a>
 	into view.
+	The visual viewport of the <a>scroll container</a>
+	(through which the <a>scrollable overflow region</a> can be viewed)
+	coincides with its padding box,
+	and is called the <dfn export>scrollport</dfn>.
 
 	<pre class=propdef>
 		Name: overflow-x, overflow-y
@@ -334,7 +338,7 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 		<dd>
 			There is no special handling of overflow, that is, it
 			may be rendered outside the box.
-			The box is not a <a>scrollable box</a>.
+			The box is not a <a>scroll container</a>.
 
 		<dt><dfn>hidden</dfn>
 		<dd>
@@ -344,7 +348,7 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 			to view the content outside the clipping region.
 			However, the content may still be scrolled programatically,
 			for example using the mechanisms defined in [[CSSOM-VIEW]],
-			and the box is therefore still a <a>scrollable box</a>.
+			and the box is therefore still a <a>scroll container</a>.
 
 		<dt><dfn>clip</dfn>
 		<dd>
@@ -357,7 +361,7 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 			which still allows programmatic scrolling,
 			''overflow: clip'' forbids scrolling entirely,
 			through any mechanism,
-			and therefore the box is not a <a>scrollable box</a>.
+			and therefore the box is not a <a>scroll container</a>.
 
 			Issue: Mozilla implements -moz-hidden-unscrollable,
 			which is similar to ''clip'',
@@ -369,7 +373,7 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 			This value indicates that
 			the content is clipped to the padding box,
 			but can be scrolled into view
-			(and therefore the box is a <a>scrollable box</a>).
+			(and therefore the box is a <a>scroll container</a>).
 			Furthermore, if the user agent uses a scrolling mechanism
 			that is visible on the screen (such as a scroll bar or a panner),
 			that mechanism should be displayed
@@ -384,7 +388,7 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 			This value indicates that
 			the box’s content is clipped to the padding box,
 			but can be scrolled into view
-			(and therefore the box is a <a>scrollable box</a>).
+			(and therefore the box is a <a>scroll container</a>).
 			However, if the user agent uses a scrolling mechanism
 			that is visible on the screen (such as a scroll bar or a panner),
 			that mechanism should only be displayed
@@ -465,7 +469,7 @@ Scrolling Origin, Direction, and Restriction</h3>
 	Due to Web-compatibility constraints
 	(caused by authors exploiting legacy bugs to surreptitiously hide content from visual readers but not search engines and/or speech output),
 	UAs must clip the <a>scrollable overflow region</a>
-	of <a>scrollable boxes</a>
+	of <a>scroll containers</a>
 	on the <a>block-start</a> and </a>inline-start</a> sides of the box
 	(thereby behaving as if they had no <a>scrollable overflow</a> on that side).