[css-grid] Add fit-content() per WG resolution

Author: fantasai

css-grid/Overview.bs

@@ -1203,7 +1203,7 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 		<dfn>&lt;auto-track-list></dfn>    = [ <<line-names>>? [ <<fixed-size>> | <<fixed-repeat>> ] ]+ <<auto-repeat>>
 		                       [ <<line-names>>? [ <<fixed-size>> | <<fixed-repeat>> ] ]+ <<line-names>>?
 
-		<dfn>&lt;track-size></dfn>         = <<track-breadth>> | minmax( <<inflexible-breadth>> , <<track-breadth>> )
+		<dfn>&lt;track-size></dfn>         = <<track-breadth>> | minmax( <<inflexible-breadth>> , <<track-breadth>> ) | fit-content( [ <<length>> | <<percentage>> ] )
 		<dfn>&lt;fixed-size></dfn>         = <<fixed-breadth>> | minmax( <<fixed-breadth>> , <<track-breadth>> ) | minmax( <<inflexible-breadth>> , <<fixed-breadth>> )
 		<dfn>&lt;track-breadth></dfn>      = <<length>> | <<percentage>> | <<flex>> | min-content | max-content | auto
 		<dfn>&lt;inflexible-breadth></dfn> = <<length>> | <<percentage>> | min-content | max-content | auto
@@ -1267,6 +1267,14 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 
 			Note: ''grid-template-rows/auto'' track sizes (and only ''grid-template-rows/auto'' track sizes)
 			can be streched by the 'align-content' and 'justify-content' properties.
+
+		<dt><dfn lt="fit-content()">fit-content( [ <<length>> | <<percentage>> ] )</dfn>
+		<dd>
+			Represents the formula
+			<code>min(''max-content'', max(''auto'', <var>argument</var>))</code>,
+			which is calculated similar to ''auto'' (i.e. ''minmax(auto, max-content)''),
+			except that the track size is clamped at <var>argument</var>
+			if it is greater than the ''auto'' minimum.
 	</dl>
 
 	In <<line-names>>, the <<custom-ident>> additionally excludes the keyword <css>span</css>.
@@ -3549,13 +3557,17 @@ Track Sizing Terminology</h3>
 		<dd>
 			If the track was sized with a ''minmax()'' function,
 			this is the first argument to that function.
-			Otherwise, it's the track's sizing function.
+			If the track was sized with a <<flex>> value or ''fit-content()'' function,
+			''auto''.
+			Otherwise, the track's sizing function.
 
 		<dt><dfn>max track sizing function</dfn>
 		<dd>
 			If the track was sized with a ''minmax()'' function,
 			this is the second argument to that function.
-			Otherwise, it's the track's sizing function.
+			If the track was sized with a ''fit-content()'' function,
+			''max-content''.
+			Otherwise, the track's sizing function.
 
 		<dt><dfn>available grid space</dfn>
 		<dd>
@@ -3889,6 +3901,9 @@ Maximize Tracks</h3>
 	If the <a>free space</a> is positive, distribute it equally to all tracks,
 	freezing tracks as they reach their <a>growth limits</a>
 	(and continuing to grow the unfrozen tracks as needed).
+	For tracks sized with ''fit-content()'',
+	first clamp the <a>growth limit</a>
+	by the track sizing function given as the ''fit-content()'' argument.
 
 	For the purpose of this step:
 	if sizing the <a>grid container</a> under a <a>max-content constraint</a>,