[css-display][various] Split out 'independent FC' from 'new FC' and rewrite definitions to be more explicit in general, and edit various specs to properly refer to it. Closes #2597. Fixes #1457 properly.

Author: tabatkins

css-align-3/Overview.bs

@@ -915,7 +915,7 @@ The 'justify-content' And 'align-content' Properties</h3>
 		<tr>
 			<th>''justify-content/normal'' Behavior
 			<td>
-				All values other than ''justify-content/normal'' force the block container to establish a new formatting context.
+				All values other than ''justify-content/normal'' force the block container to [=establish an independent formatting context=].
 
 				For table cells, the behavior of the ''justify-content/normal'' depends on the computed value of 'vertical-align':
 				''vertical-align/top'' makes it behave as ''start'',

css-contain-1/Overview.bs

@@ -217,7 +217,7 @@ Layout Containment</h3>
 	layout containment has no effect.
 	Otherwise, giving an element <dfn export>layout containment</dfn> has the following effects:
 
-	1. The element establishes a new <a>formatting context</a>.
+	1. The element [=establishes an independent formatting context=].
 
 	2. If a <a>fragmentation context</a> participates in <a>layout containment</a>,
 		the first element with <a>layout containment</a>
@@ -352,7 +352,7 @@ Paint Containment</h3>
 		<span class=note>This is as if to ''overflow: visible'' was changed to ''overflow: clip'' at used value.</span>
 	2. The element acts as a containing block for absolutely positioned and fixed positioned descendants.
 	3. The element creates a <a>stacking context</a>.
-	4. The element establishes a new <a>formatting context</a>.
+	4. The element [=establishes an independent formatting context=].
 
 	Possible optimizations that can be enabled by <a>paint containment</a> include (but are not limited to):
 

css-display-3/Overview.bs

@@ -1030,20 +1030,37 @@ Appendix A: Glossary</h2>
 			lays out boxes according to the <a>flex layout</a> rules [[CSS3-FLEXBOX]],
 			whereas a <a>block formatting context</a>
 			lays out boxes according to the block-and-inline layout rules [[CSS2]].
+			Additionally, some types of [=formatting contexts=] interleave and co-exist:
+			for example, an [=inline formatting context=] exists within
+			and interacts with the [=block formatting context=]
+			of the element that establishes it,
+			and a [=ruby container=] overlays a [=ruby formatting context=]
+			over the [=inline formatting context=] in which its [=ruby base container=] participates.
 
-			When a box establishes a <em>new</em> [=formatting context=]
+			A box either establishes a new [=independent formatting context=]
+			or continues the [=formatting context=] of its containing block.
+			In some cases,
+			it might additionally establish a new (non-independent) co-existing formatting context.
+			Unless otherwise specified, however,
+			establishing a new <a>formatting context</a>
+			creates an <a>independent formatting context</a>.
+			The type of formatting context established by the box
+			is determined by its <a>inner display type</a>.
+			E.g. a <a>grid container</a> establishes a new <a>grid formatting context</a>,
+			a <a>ruby container</a> establishes a new <a>ruby formatting context</a>,
+			and a <a>block container</a> can establish a new <a>block formatting context</a>
+			and/or a new <a>inline formatting context</a>.
+			See the 'display' property.
+
+		<dt><dfn>independent formatting context</dfn>
+		<dd>
+			When a box establishes an independent formatting context
 			(whether that [=formatting context=] is of the same type as its parent or not),
 			it essentially creates a new, independent layout environment:
 			except through the sizing of the box itself,
 			the layout of its descendants is (generally)
 			not affected by the the rules and contents of
 			the [=formatting context=] outside the box, and vice versa.
-			However, certain types of [=formatting context=] interleave and co-exist:
-			for example, an [=inline formatting context=] exists within
-			and interacts with the [=block formatting context=]
-			of the element that establishes it,
-			and a [=ruby container=] overlays a [=ruby formatting context=]
-			over the [=inline formatting context=] in which its [=ruby base container=] participates.
 
 			<p class="example">
 				For example, in a <a>block formatting context</a>,
@@ -1058,10 +1075,38 @@ Appendix A: Glossary</h2>
 				As another example, margins do not collapse across [=formatting context=] boundaries.
 
 			<p class="note">
-				Exclusions are able to affect content across [=formatting context=] boundaries.
+				Exclusions are able to affect content across [=independent formatting context=] boundaries.
 				(At time of writing, they are the only layout feature that can.)
 				[[CSS3-EXCLUSIONS]]
 
+			Certain properties can force a box to
+			<dfn lt="establish an independent formatting context|
+			         establishes an independent formatting context|
+			         establishing an independent formatting context|
+			         established an independent formatting context">
+				establish an independent formatting context</dfn>
+			in cases where it wouldn't ordinarily.
+			For example,
+			making a box <a>out-of-flow</a>
+			causes it to <a>blockify</a> as well as to [=establish an independent formatting context=].
+			As another example,
+			certain values of the 'contain' property
+			can cause a box to <a>establish an independent formatting context</a>.
+			Turning a block into a <a>scroll container</a>
+			will cause it to <a>establish an independent formatting context</a>;
+			however turning a <a>subgrid</a> into a <a>scroll container</a> will not--
+			it continues to act as a subgrid,
+			with its contents participating in the layout of its parent <a>grid container</a>.
+
+			A [=block box=] that [=establishes an independent formatting context=]
+			establishes a new [=block formatting context=] for its contents.
+			In most other cases,
+			forcing a box to [=establish an independent formatting context=] is a no-op--
+			either the box already establishes an <a>independent formatting context</a>
+			(e.g. <a>flex containers</a>),
+			or it's not possible to establish a totally independent new formatting context on that type of box
+			(e.g. non-replaced <a>inline boxes</a>).
+
 		<dt><dfn>block formatting context</dfn>
 		<dt><dfn>inline formatting context</dfn>
 		<dd>
@@ -1259,6 +1304,11 @@ Appendix C: Box Construction Guidelines for Spec Authors</h2>
 		can't directly contain <a>block-level</a> boxes;
 		any boxes generated within such an element
 		must be <a>inline-level</a>.
+	* Boxes that fundamentally cannot establish an [=independent formatting context=]
+		(such as non-replaced inlines)
+		must not be asked to [=establish an independent formatting context=].
+		Blockify them first,
+		or otherwise change their box type to one that can establish an [=independent formatting context=].
 
 <h2 class=no-num id="acknowledgments">
 Acknowledgments</h2>
@@ -1340,7 +1390,7 @@ Changes</h2>
 	* Clarified interaction of various box tree fixups.
 		(<a href="https://drafts.csswg.org/css-display-3/issues-wd-2017#issue-38">Issue 38</a>,
 		<a href="https://drafts.csswg.org/css-display-3/issues-wd-2017#issue-48">Issue 48</a>)
-	* Added the definition of <a>becoming a formatting context</a>.
+	* Added the definition of “becoming a formatting context”.
 	* Miscellaneous minor fixes and minor clarifications.
 
 	A <a href="https://drafts.csswg.org/css-display-3/issues-wd-2017">Disposition of Comments</a> is also available.

css-flexbox-1/Overview.bs

@@ -638,8 +638,7 @@ Flex Items</h2>
 		Its contents will however inherit styles (such as font settings) from the flex container.
 	</div>
 
-	A <a>flex item</a> establishes a new formatting context for its contents.
-	The type of this formatting context is determined by its 'display' value, as usual.
+	A <a>flex item</a> [=establishes an independent formatting context=] for its contents.
 	However, flex items themselves are <dfn>flex-level</dfn> boxes, not block-level boxes:
 	they participate in their container's flex formatting context,
 	not in a block formatting context.

css-grid-1/Overview.bs

@@ -1034,8 +1034,8 @@ Grid Items</h2>
 
 <h3 id="grid-item-display">
 	Grid Item Display</h3>
-	A <a>grid item</a> establishes a new formatting context for its contents.
-	The type of this formatting context is determined by its 'display' value, as usual.
+
+	A <a>grid item</a> [=establishes an independent formatting context=] for its contents.
 	However, grid items are <dfn>grid-level</dfn> boxes, not block-level boxes:
 	they participate in their container's <a>grid formatting context</a>,
 	not in a block formatting context.

css-inline-3/Overview.bs

@@ -1107,7 +1107,7 @@ Short paragraphs with initial letters</h4>
 	</figure>
 
 	If the subsequent block starts with an initial letter,
-	establishes a new formatting context,
+	establishes an [=independent formatting context=],
 	or specifies 'clear' in the initial letter’s containing block’s start direction,
 	then it must clear the previous block’s initial letter.
 

css-multicol-1/Overview.bs

@@ -778,8 +778,8 @@ Spanning columns</h2>
 			ancestor in the same block formatting context.
 			Content in the normal flow that appears before the
 			element is automatically balanced across all columns in the immediately preceding column row before the
-			element appears. 
-			The element establishes a new <a>formatting context</a>.
+			element appears.
+			The element [=establishes an independent formatting context=].
 
 			Note: Whether the element establishes a new <a>formatting context</a>
 			does not depend on whether the element is a descendent of a multicol or not.

css-overflow-3/Overview.bs

@@ -385,7 +385,7 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 
 	If the computed value of 'overflow' on a <a>block box</a>
 	is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
-	it creates a new <a>block formatting context</a> for its content.
+	it [=establishes an independent formatting context=] for its contents.
 
 
 <h3 id="static-media">

css-rhythm-1/Overview.bs

@@ -181,7 +181,7 @@ Specifying the Step Size: the 'block-step-size' property {#block-step-size}
 	Negative <<length>> values are invalid.
 
 	Values other than ''block-step-size/none''
-	cause the box to establish a new formatting context.
+	cause the box to [=establish an independent formatting context=].
 
 	In situations where margins <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>,
 	only the box’s own margin is considered