[css-overflow] Flesh out the overflow values, and include clipped

Author: frivoal

css-overflow/Overview.bs

@@ -377,7 +377,7 @@ Introduction</h2>
 
 	<pre class=propdef>
 		Name: overflow-x, overflow-y
-		Value: ''visible'' | ''hidden'' | ''scroll'' | ''auto''
+		Value: ''visible'' | ''hidden'' | ''clipped'' | ''scroll'' | ''auto'' 
 		Initial: ''visible''
 		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
@@ -396,7 +396,7 @@ Introduction</h2>
 
 	<pre class=propdef>
 		Name: overflow
-		Value: ''visible'' | ''hidden'' | ''scroll'' | ''auto''
+		Value: ''visible'' | ''hidden'' | ''clipped'' | ''scroll'' | ''auto'' 
 		Initial: see individual properties
 		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
@@ -434,15 +434,66 @@ Introduction</h2>
 			may be rendered outside the block container.
 		</dd>
 		<dt><dfn>hidden</dfn>
+		<dd>This value indicates that
+		the content is clipped and that no scrolling user interface should be provided by the UA
+		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]].
+		<dt><dfn>clipped</dfn>
+		<dd>Like ''hidden'',
+		this value indicates that
+		the content is clipped
+		and that no scrolling user interface should be provided by the UA
+		to view the content outside the clipping region.
+		In addition, unlike ''overflow: hidden''
+		which still allows programmatic scrolling,
+		''overflow: clipped'' forbids scrolling entirely,
+		through any mechanism.
+		
+		Issue: Mozilla implements -moz-hidden-unscrollable,
+		which is similar to ''clipped'',
+		except that it does not cause the element to establish a BFC.
+		Should we match that?
 		<dt><dfn>scroll</dfn>
+		<dd>This value indicates that the content is clipped
+		and that 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 for a box
+		whether or not any of its content is clipped.
+		This avoids any problem with scrollbars appearing
+		and disappearing in a dynamic environment.
+		When this value is specified and the target medium is ''print'',
+		overflowing content may be printed. 
 		<dt><dfn>auto</dfn>
-		<dd>
-			<p class="issue">
-			Move material from [[CSS21]] and [[CSS3BOX]] here.
-			</p>
-		</dd>
+		<dd>The behavior of the ''overflow/auto'' value is user agent-dependent,
+		but should cause a scrolling mechanism to be provided for overflowing boxes. 
 	</dl>
 
+	Even if 'overflow' is set to ''overflow/visible'',
+	content may be clipped to a UA's document window by the native operating environment. 
+
+	If the computed value of 'overflow' is not ''overflow/visible'',
+	the element creates a block formatting context.
+
+	UAs must apply the 'overflow' property
+	set on the root element to the viewport.
+	HTML UAs must instead apply the ‘overflow’ property
+	from the <{body}> element to the viewport
+	if the value on the root element is ''visible''.
+	The ''visible'' value when used for the viewport
+	must be interpreted as ''overflow/auto''.
+	The element from which the value is propagated
+	must have a used value for 'overflow' of ''visible''.
+
+	In the case of a scrollbar being placed on an edge of the element's box,
+	it should be inserted between the inner border edge
+	and the outer padding edge.
+	Any space taken up by the scrollbars should be
+	taken out of (subtracted from the dimensions of)
+	the containing block formed by the element with the scrollbars. 
+
+	Issue: import examples from [[CSS3-BOX]].
+
 	<p class="issue">
 		Explain which directions allow scrolling and which don't,
 		as a function of 'direction'