[css-position-3] Add definition of static position rectangle.

Author: fantasai

css-position-3/Overview.bs

@@ -710,6 +710,66 @@ Absolute Positioning</h3>
 	after [[#abspos-layout|layout]].
 	(Otherwise the [=resolved value=] is the [=used value=] described above.)
 
+
+<h4 id="staticpos-rect">
+Resolving Automatic Insets</h4>
+
+	When both [=inset properties=] in a given axis are ''left/auto'',
+	they are resolved in reference to the box’s <dfn export>static position rectangle</dfn>,
+	whose edges represent the [=static position=] of the box
+	from each side of its [=containing block=].
+	This [=static position rectangle=]
+	and the [=static positions=] to which it corresponds
+	are defined by the layout model
+	the box would have had if its 'position' were ''position/static''
+	(and its 'float' and 'clear' values were ''float/none'').
+	(Note that this may require assuming a different hypothetical value
+	for 'display' as well.)
+
+	<dl>
+		<dt>Block Layout
+		<dd>
+			The [=static positions=] of a [=block-level box=]
+			are defined in [[CSS2]] Chapter 10.
+			The [=static position rectangle=] is a zero-thickness rectangle spanning between
+			the inline-axis sides of the box’s [=static-position containing block=]
+			(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2&sect;10.3.7</a>);
+			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>).
+
+		<dt>Inline Layout
+		<dd>
+			The [=static positions=] of an [=inline-level box=]
+			are defined in [[CSS2]] Chapter 10.
+			The [=static position rectangle=] is a zero-thickness rectangle spanning between
+			the [=line-over=]/[=line-under=] sides of the [=line box=]
+			that would have contained its “hypothetical box”
+			(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2&sect;10.3.7</a>);
+			and positioned at its [=inline-start=] [=static position=].
+
+		<dt>Flex Layout
+		<dd>
+			The [=static position rectangle=] of the child of a [=flex container=]
+			corresponds to the [=content edges=] of the [=flex container=].
+			See <a href="https://www.w3.org/TR/css-flexbox-1/#abspos-items">static position of a grid container child</a> in [[!CSS-FLEXBOX-1]].
+
+		<dt>Grid Layout
+		<dd>
+			By default, the [=static position rectangle=] of the child of a [=grid container=]
+			corresponds to the [=content edges=] of the [=grid container=].
+			However, if that [=grid container=] also establishes
+			the box’s actual [=containing block=],
+			then the [=grid area=] specified by the [=grid-placement properties=]
+			establishes its [=static position rectangle=] instead.
+			See the <a href="https://www.w3.org/TR/css-grid-1/#static-position">static position of a grid container child</a> in [[!CSS-GRID-1]].
+	</dl>
+
+	For the purposes of calculating the [=static position rectangle=],
+	the [=containing block=] of [=fixed positioned elements=]
+	is the [=initial containing block=] instead of the [=viewport=],
+	and all [=scroll containers=] should be assumed
+	to be scrolled to their [=initial scroll position=].
+
 <h4 id="abspos-breaking" oldids="breaking">
 Fragmenting Absolutely-positioned Elements</h4>
 
@@ -1592,4 +1652,4 @@ and entering in sensitive information that they page can then capture.
 User agents must ensure that their native dialogs are positioned
 in ways that the page cannot emulate;
 in particular,
-that at least some of the dialog is outside the "poisoned pixels" that web content can paint to.
+that at least some of the dialog is outside the "poisoned pixels" that web content can paint to.