[css-flexbox-1] Simply use cross-axis content edges for static position rectangle, matching Grid. w3c#5843

Also move the historical note to CSS Positioned Layout.

Author: fantasai

css-flexbox-1/Overview.bs

@@ -739,14 +739,16 @@ Absolutely-Positioned Flex Children</h3>
 	As it is out-of-flow,
 	an absolutely-positioned child of a <a>flex container</a> does not participate in flex layout.
 
-	The <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">static position</a>
+	The [=cross-axis=] edges of the [=static-position rectangle=]
 	of an absolutely-positioned child of a <a>flex container</a>
-	is determined such that the child is positioned
-	as if it were the sole <a>flex item</a> in the <a>flex container</a>,
+	are the [=content edges=] of the [=flex container=]
+	The [=main-axis=] edges of the [=static-position rectangle=]
+	are where the [=margin edges=] of the child would be positioned
+	if it were the sole <a>flex item</a> in the <a>flex container</a>,
 	assuming both the child and the flex container
 	were fixed-size boxes of their used size.
-	For this purpose,
-	''margin/auto'' margins are treated as zero.
+	(For this purpose,
+	''margin/auto'' margins are treated as zero.)
 
 	<wpt>
 		abspos/abspos-autopos-htb-ltr.html
@@ -757,28 +759,10 @@ Absolutely-Positioned Flex Children</h3>
 		abspos/abspos-autopos-vrl-rtl.html
 	</wpt>
 
-	<div class="note">
-		In other words,
-		the <a>static-position rectangle</a> of an absolutely-positioned child of a <a>flex container</a>
-		is the <a>flex container's</a> content box,
-		where the <dfn>static-position rectangle</dfn> is the <a>alignment container</a>
-		used to determine the static-position offsets of an absolutely-positioned box.
-
-		(In block layout the static position rectangle corresponds to the position of the “hypothetical box”
-		described in <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2.1&sect;10.3.7</a>.
-		Since it has no alignment properties,
-		CSS2.1 always uses a <a>block-start</a> <a>inline-start</a> alignment
-		of the absolutely-positioned box within the <a>static-position rectangle</a>.
-		Note that this definition will eventually move to the CSS Positioning module.)
-	</div>
-
 	<div class='example'>
 		The effect of this is that if you set, for example,
 		''align-self: center;'' on an absolutely-positioned child of a <a>flex container</a>,
 		auto offsets on the child will center it in the <a>flex container's</a> <a>cross axis</a>.
-
-		However, since the absolutely-positioned box is considered to be “fixed-size”,
-		a value of ''align-self/stretch'' is treated the same as ''align-self/flex-start''.
 	</div>
 
 <h3 id='item-margins'>
@@ -3685,6 +3669,14 @@ Changes since the <a href="https://www.w3.org/TR/2018/CR-css-flexbox-1-20181119/
 	Significant changes include:
 
 	<ul>
+		<li id="change-2018-static-pos-align-content">
+			Use the [=content edges=] of the [=flex container=]
+			for determining the [=static position rectangle=] of flex container children,
+			rather than doing extra layout work.
+			This will result in ignoring 'align-content' on the flex container,
+			but allow 'align-self' on the box itself to still take effect.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/5843">Issue 5843</a>)
+
 		<li id="change-2018-correctly-ignore-auto">
 			Correctly ignore ''width/auto'' [=preferred sizes=]
 			in both min <em>and</em> max size contribution calculations.

css-position-3/Overview.bs

@@ -793,6 +793,12 @@ Resolving Automatic Insets</h4>
 			and positioned at its [=block-start=] [=static position=]
 			(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">CSS2&sect;10.6.4</a>).
 
+			Note: In block layout the static-position rectangle corresponds to the position of the “hypothetical box”
+			described in <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2.1&sect;10.3.7</a>.
+			Since it has no alignment properties,
+			CSS2.1 always uses a <a>block-start</a> <a>inline-start</a> alignment
+			of the absolutely-positioned box within the <a>static-position rectangle</a>.
+
 		<dt>Inline Layout
 		<dd>
 			The [=static positions=] of an [=inline-level box=]