[css-position-4] Define the 'scrollable containing block'. w3c#10861

Author: tabatkins

css-position-4/Overview.bs

@@ -334,6 +334,48 @@ It is recommended that UAs use [=out-of-band outlines=],
 as making outlines easily visible
 is an important accessibility feature.
 
+
+Scrollable Containing Block {#scrollable-cb}
+===========================
+
+When an element
+generates an [=absolute positioning containing block=]
+for its descendants,
+that [=containing block=] is generated from the element's [=padding box=] edges.
+If that element is a [=scroll container=],
+this corresponds to the dimensions of the [=scrollport=],
+and does not shift as the scroll container is scrolled.
+In other words,
+it's "outside of" the scrollable area.
+
+Sometimes, however, you instead want a [=containing block=]
+based on the "inside" of the scroll container,
+sized according to the scrollable contents
+and shifting as the container is scrolled.
+
+Whenever a [=scroll container=] generates an [=absolute positioning containing block=],
+it addition to the normal [=containing block=] it generates,
+it also generates a <dfn export>scrollable containing block</dfn>.
+This is sized to match the [=padding box=] edges
+of the scrollable area,
+and is treated as being "inside" the scrollable area,
+so it moves as the [=scroll container=] is scrolled.
+
+Note: This is not <em>quite</em> the [=scrollable overflow rectangle=];
+in particular, it doesn't include the effects of transformed elements
+or absolutely positioned elements.
+
+In [=containing block chains=],
+the [=scrollable containing block=] generated by an element
+is considered a child of the element's normal [=containing block=],
+and a parent of any containing blocks generated by its descendants.
+
+The [=scrollable containing block=] is not used by default,
+but certain CSS features
+(such as 'position-area')
+can specify that they use it instead of an element's normal [=containing block=].
+
+
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