[css-grid] Percentage tracks are treated as ''auto'' for intrinsic sizing of the grid container, and then resolved against that resulting size, per WG resolution. <https://lists.w3.org/Archives/Public/www-style/2017Feb/0061.html> #509

Author: fantasai

css-grid/Overview.bs

@@ -1400,11 +1400,10 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 			If the size of the <a>grid container</a>
 			depends on the size of its tracks,
 			then the <<percentage>>
-			must be treated as <a value for=width>auto</a>.
-			The UA <em>may</em> adjust the intrinsic size contributions of the track
-			to the size of the <a>grid container</a>
-			and increase the final size of the track
-			by the minimum amount that would result in honoring the percentage.
+			must be treated as <a value for=width>auto</a>,
+			for the purpose of calculating the intrinsic sizes of the <a>grid container</a>
+			and then resolve against that resulting <a>grid container</a> size
+			for the purpose of laying out the <a>grid</a> and its items.
 
 		<dt><dfn><<flex>></dfn>
 		<dd>
@@ -4246,6 +4245,43 @@ Major Changes</h4>
 			(<a href="https://github.com/w3c/csswg-drafts/issues/523">Issue #523</a>)
 			See <a href="#grid-item-sizing"></a>
 			(vs. <a href="https://www.w3.org/TR/2016/CR-css-grid-1-20160929/#grid-item-sizing">original</a>).
+
+		<li id="change-2016-auto-percent-track">
+			Changed the behavior of <<pecentage>> tracks
+			inside a <a>grid container</a> whose size depends on the size of those tracks
+			to match implementations by
+			contributing their dimensions sized as ''grid-template-columns/auto''
+			and subsequently resolve the percentage against
+			the resulting <a>grid container</a> size
+			rather than being treated exactly as an ''grid-template-columns/auto'' track
+			or having their size and that of the <a>grid container</a>
+			increased from an ''grid-template-columns/auto'' size
+			in order to honor the percentage without overflow.
+			This will frequently result in tracks overflowing
+			the <a>grid container</a>
+			and in the contents of tracks overflowing the tracks
+			when <<percentage>> sizes are used
+			in <a>fit-content</a>-sized <a>grid containers</a>
+			such as ''width/auto''-sized inline or floated <a>grid containers</a>.
+			(To avoid this problem, use <<flex>> units instead,
+			which are intended to maintain their ratios and not overflow
+			when the grid is intrinsically-sized.)
+			<blockquote>
+				<p>
+					If the size of the <a>grid container</a>
+					depends on the size of its tracks,
+					then the <<percentage>>
+					must be treated as <a value for=width>auto</a>
+					<ins>for the purpose of calculating the intrinsic sizes of the <a>grid container</a>
+					and then resolve against that size for the purpose of laying out the grid and its items</ins>.
+					<del>
+						The UA <em>may</em> adjust the intrinsic size contributions of the track
+						to the size of the <a>grid container</a>
+						and increase the final size of the track
+						by the minimum amount that would result in honoring the percentage.
+					</del>
+				</p>
+			</blockquote>
 	</ul>
 
 <h4 id="fixes-2016">