[css-grid] Define that empty auto-fit tracks get *collapsed*, not dropped - collapsed tracks are forced to 0px, and collapsed gutters coincide. For issue w3c#172.

Author: tabatkins

css-grid/Overview.bs

@@ -985,16 +985,6 @@ Non-children Grid Items</h3>
 		and missing names are treated as <a value for="<grid-line>">auto</a>.
 -->
 
-<h3 id="visibility-collapse">
-	Collapsed Grid Items: the 'visibility' property</h3>
-
-	<p class='issue'>
-		We want the ability to collapse grid tracks
-		(similar to <a href="https://www.w3.org/TR/css3-flexbox/#visibility-collapse">collapsing flex items</a>
-		 or <a href="https://www.w3.org/TR/CSS21/tables.html#dynamic-effects">table rows/columns</a>),
-		but we're not sure exactly how to do it.
-		Ideas welcome, please <a href="mailto:www-style@w3.org?Subject=%5Bcss-grid%5D%20Collapsing%20Grid%20Tracks">post them to www-style@w3.org</a>.
-
 <h3 id='order-property'>
 Reordered Grid Items: the 'order' property</h3>
 
@@ -1486,11 +1476,15 @@ Repeat-to-fill: ''auto-fill'' and ''auto-fit'' repetitions</h5>
 
 	The <dfn value for="repeat()">auto-fit</dfn> keyword behaves the same as ''auto-fill'',
 	except that after <a href="#auto-placement-algo">grid item placement</a>
-	any empty repetitions are dropped.
+	any empty repeated tracks are <a>collapsed</a>.
 	An empty track is one with no in-flow grid items placed into or spanning across it.
-	(This can result in <em>all</em> tracks being dropped,
+	(This can result in <em>all</em> tracks being <a>collapsed</a>,
 	if they're all empty.)
 
+	A <dfn local-lt=collapse>collapsed track</dfn> is treated as having
+	a fixed <a>track sizing function</a> of ''0px'',
+	and the <a>gutters</a> on either side of it <a lt="collapsed gutter">collapse</a>.
+
 	For the purpose of finding the number of auto-repeated tracks,
 	the UA must floor the track size to a UA-specified value
 	to avoid division by zero.
@@ -3234,6 +3228,14 @@ Gutters: the 'grid-column-gap', 'grid-row-gap', and 'grid-gap' properties</h3>
 	(In particular, there is no <a>gutter</a> between the first/last track of the <a>implicit grid</a>
 	and the “auto” lines in the <a>augmented grid</a>.)
 
+	When a <a>collapsed track</a>’s gutters <dfn lt="collapsed gutter">collapse</dfn>,
+	they coincide exactly--
+	the two gutters overlap so that their start and end edges coincide.
+	If one side of a <a>collapsed</a> track does not have a gutter
+	(e.g. if it is the first or last track of the <a>implicit grid</a>),
+	then collapsing its gutters results in no gutter
+	on either “side” of the <a>collapsed track</a>.
+
 	<pre class='propdef'>
 	Name: grid-gap
 	Value: <<'grid-row-gap'>> <<'grid-column-gap'>>?