[css-overflow-4] Define longhands for 'overflow-clip-margin' #7245

Author: fantasai

css-overflow-4/Overview.bs

@@ -184,6 +184,74 @@ Scrolling and Clipping Overflow</h2>
 
 	Issue: Copy [[css-overflow-3#overflow-properties|Level 3 content]] when final.
 
+<h3 id="overflow-clip-margin">
+Expanding Clipping Bounds: the 'overflow-clip-margin-*' properties</h3>
+
+	<pre class="propdef shorthand">
+	Name: overflow-clip-margin-top, overflow-clip-margin-right, overflow-clip-margin-bottom, overflow-clip-margin-left, overflow-clip-margin-block-start, overflow-clip-margin-inline-start, overflow-clip-margin-block-end, overflow-clip-margin-inline-end
+	Value: <<visual-box>> || <<length [0,∞]>>
+	Initial: ''0px''
+	Inherited: no
+	Applies to: boxes to which 'overflow' applies
+	Computed value: the computed <<length>> and a <<visual-box>> keyword
+	Animation type: per computed value if the <<visual-box>> values match; otherwise discrete
+	</pre>
+
+	<pre class="propdef shorthand">
+	Name: overflow-clip-margin, overflow-clip-margin-inline, overflow-clip-margin-block
+	Value: <<visual-box>> || <<length [0,∞]>>
+	Initial: ''0px''
+	Inherited: no
+	Applies to: boxes to which 'overflow' applies
+	</pre>
+
+	These properties and their shorthands
+	define the <dfn export>overflow clip edge</dfn> of the box,
+	i.e. precisely <em>how far</em> outside its bounds
+	the box’s content is allowed to paint
+	before being clipped
+	by effects (such as ''overflow: clip'', above)
+	that are defined to clip to the box’s [=overflow clip edge=].
+	The [=shorthand=]/[=longhand=] relationships are as for 'margin',
+	but note that the [=shorthands=] have a restricted syntax.
+
+	Values are defined as follows:
+
+	<dl dfn-type=value dfn-for=overflow-clip-margin>
+		: <dfn><<visual-box>></dfn>
+		::
+			Specifies the box edge to use as the [=overflow clip edge=] origin,
+			i.e. when the specified offset is zero.
+
+			If omitted,
+			defaults to ''overflow-clip-margin/padding-box'' on non-[=replaced=] elements,
+			or ''overflow-clip/content-box'' on [=replaced=] elements.
+
+			ISSUE(7144): Application of 'overflow-clip-margin' to [=replaced elements=]
+			is still being worked out.
+
+		: <dfn><<length [0,∞]>></dfn>
+		::
+			The specified offset dictates
+			how much the [=overflow clip edge=] is expanded from
+			the specified box edge
+			Negative values are invalid.
+			Defaults to zero if omitted.
+	</dl>
+
+	The [=overflow clip edge=] is shaped in the corners
+	exactly the same way as an <a spec=css-backgrounds-3>outer box-shadow</a>
+	with a spread radius of the same cumulative offset
+	from the box’s [=border edge=].
+	See [[css-backgrounds-3#corner-shaping]]
+	and [[css-backgrounds-3#shadow-shape]],
+	noting in particular the formula for outsets beyond the [=border edge=].
+
+	Note: This property has no effect on boxes
+	with ''overflow: hidden'' or ''overflow: scroll'',
+	which are not defined to use the [=overflow clip edge=].
+
+
 <h2 id="auto-ellipsis">
 Automatic Ellipses</h2>