[css-sizing-4] First draft of c-i-s:auto. #5668

Author: tabatkins

css-sizing-4/Overview.bs

@@ -487,7 +487,7 @@ Overriding Contained Intrinsic Sizes: the 'contain-intrinsic-*' properties</h3>
 
 	<pre class=propdef>
 	Name: contain-intrinsic-width, contain-intrinsic-height, contain-intrinsic-block-size, contain-intrinsic-inline-size
-	Value: none | <<length>>
+	Value: none | <<length>> | auto <<length>>
 	Initial: none
 	Inherited: no
 	Applies to: elements with [=size containment=]
@@ -517,6 +517,12 @@ Overriding Contained Intrinsic Sizes: the 'contain-intrinsic-*' properties</h3>
 
 		: <dfn><<length>></dfn>
 		:: The corresponding axis has an [=explicit intrinsic inner size=] of the specified <<length>>.
+
+		: <dfn>auto <<length>></dfn>
+		:: The corresponding axis has an [=explicit intrinsic inner size=]
+			of its [=last remembered size=].
+			If a [=last remembered size=] does not yet exist,
+			instead use the specified <<length>>.
 	</dl>
 
 	If an element has an [=explicit intrinsic inner size=] in an axis,
@@ -544,7 +550,7 @@ Overriding Contained Intrinsic Sizes: the 'contain-intrinsic-*' properties</h3>
 
 	<pre class="propdef shorthand">
 	Name: contain-intrinsic-size
-	Value: [ none | <<length>> ]{1,2}
+	Value: [ none | <<length>> | auto <<length>> ]{1,2}
 	</pre>
 
 	'contain-intrinsic-size' is a shorthand property
@@ -555,6 +561,53 @@ Overriding Contained Intrinsic Sizes: the 'contain-intrinsic-*' properties</h3>
 	If only one value is given,
 	it applies to both properties.
 
+<h4 id='last-remembered' dfn lt="last remembered size" export>
+Last Remembered Size</h4>
+
+	[=Size containment=] is very valuable for ensuring a page can render efficiently,
+	restricting the scope of layout work that can happen
+	as a result of an element changing its rendering.
+	However, it's also very restrictive for the author,
+	requiring them to correctly predict what the size of the element will be;
+	if this guess is incorrect,
+	even slightly,
+	it can cause unsightly scrollbars or accidentally-hidden content.
+
+	The ''contain-intrinsic-size: auto'' value allows a middle-ground:
+	it lays out the element and records its sizes
+	<em>just before [=size containment=] begins to apply</em>,
+	then remembers this value
+	and uses it from then on,
+	offering a <em>probably accurate</em> size based on an actual layout of the contents
+	while still offering the performance benefits of [=size containment=]
+	from then onward.
+
+	<div algorithm="last remembered size">
+		The [=last remembered size=] of an element is determined by:
+
+		* At the time that ResizeObserver events are determined and delivered,
+			if an element has ''contain-intrinsic-size: auto''
+			but does not have [=size containment=],
+			record its current inner dimensions
+			as its [=last remembered size=].
+
+		* If the element is being rendered for the first time,
+			or has just switched from not being rendered to being rendered
+			(for example, it was previously [=skipped contents|skipped=] due to being off-screen),
+			then, even if it has [=size containment=],
+			it must be laid out as if it does <em>not</em> have [=size containment=],
+			and the resulting inner dimensions
+			must be recorded as its [=last remembered size=].
+
+			(It is then rendered as usual,
+			applying [=size containment=],
+			but also using this [=last remembered size=]
+			due to ''contain-intrinsic-size: auto''.)
+
+		An element might not have a [=last remembered size=],
+		if it has not yet been rendered.
+	</div>
+
 <h4 id='cis-scrollbars'>
 Interaction With ''overflow: auto''</h4>