[css-position-4] Correct the definition of the 'normal' CB of a scroller (it's inside the scrollable area, not outside). Add terms for the three relevant CBs.

Author: tabatkins

css-position-4/Overview.bs

@@ -292,6 +292,7 @@ with its top left corner at the canvas origin.
 						given the child,
 						|line box|,
 						and |canvas|.
+				</dl>
 
 			: If |root| is an [=inline-level=] [=block box|block=] or [=table wrapper box=]
 			:: [=Paint a stacking container=]
@@ -336,51 +337,78 @@ as making outlines easily visible
 is an important accessibility feature.
 
 
+<!-- Big Text: scroll cb
+
+ ███▌   ███▌  ████▌   ███▌  █▌    █▌         ███▌  ████▌
+█▌  █▌ █▌  █▌ █▌  █▌ █▌  █▌ █▌    █▌        █▌  █▌ █▌  █▌
+█▌     █▌     █▌  █▌ █▌  █▌ █▌    █▌        █▌     █▌  █▌
+ ███▌  █▌     ████▌  █▌  █▌ █▌    █▌        █▌     █████
+    █▌ █▌     █▌▐█   █▌  █▌ █▌    █▌        █▌     █▌  █▌
+█▌  █▌ █▌  █▌ █▌ ▐█  █▌  █▌ █▌    █▌        █▌  █▌ █▌  █▌
+ ███▌   ███▌  █▌  █▌  ███▌  █████ █████      ███▌  ████▌
+-->
+
 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.
-
-This also applies to the [=root scroller=];
-a [=scrollable containing block=] that corresponds
-to the padding edges of the entire document
-is generated,
-and is considered a child of the [=initial containing block=].
-
-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=].
+When a [=scroll container=] establishes an [=absolute positioning containing block=]
+for an [=absolutely positioned box=],
+one of three possible [=containing blocks=] are used:
+
+: <dfn export>fixed containing block</dfn>
+:: The [=fixed containing block=] of a [=scroll container=]
+	corresponds to the [=scrollport=],
+	i.e. the inner [=padding box=] edges of the [=scroll container=],
+	but scrolling with the outer context, not the [=scroll container=]’s contents.
+
+	The [=fixed containing block=] established by the document
+	is the [=fixed positioning containing block=].
+
+: <dfn export>local containing block</dfn>
+:: The [=local containing block=] of a [=scroll container=]
+	corresponds to the [=padding box=] edges of the [=scroll container=],
+	but is affixed to the [=scrollable overflow area=],
+	and scrolls with the [=scroll container=]’s contents.
+
+	The [=local containing block=] established by the document
+	is the [=initial containing block=].
+
+: <dfn export>scrollable containing block</dfn>
+:: The [=scrollable containing block=] of a [=scroll container=]
+	corresponds to the [=padding edge=] of
+	the [=scroll container=]’s [=scrollable overflow area=],
+	i.e. the outer edge of the [=padding=] surrounding its content
+	when determining the extent of its [=scrollable overflow area=].
+	See [[css-overflow-3#scrollable]].
+	In all cases the [=scrollable containing block=]
+	at least encompasses the [=local containing block=].
+
+	Note: This includes floats,
+	but ignores absolutely positioned descendants,
+	content overflowing descendant boxes,
+	and the effects of relative positioning and transforms,
+	which do otherwise extend the scrollable area to display them.
+
+	The [=scrollable containing block=] established by the document
+	is the union of the [=initial containing block=] and
+	the [=margin box=] of the [=root box=] itself.
+
+Note: While all of these are in some sense corresponding to the [=padding box=],
+they do not coincide for boxes with [=scrollable overflow=].
+
+ISSUE: Figure out the exact concepts needed for top layer.
+They probably want to do something a tiny bit different,
+specific to their layer.
+
+Unless otherwise specified,
+absolutely positioned boxes use the [=local containing block=].
+Certain CSS features can specify a different [=containing block=].
+For example, [=fixed positioned boxes=] typically use the document’s [=fixed containing block=],
+and 'position-area' values other than ''position-area/none''
+can opt absolutely positioned boxes into the [=scrollable containing block=].
+
+Note: There is not currently any way to refer to a [=fixed containing block=]
+other than the [=fixed positioning containing block=],
+but one might be added in the future.
 
 
 <!-- Big Text: top